the 5th mission in my arc is tough because of the custom group. If you have a kin it will help. It is not a super tough arc, it is really quite simple. But it can be a challenge.
Arc 69582.
Let me know what you think. I am going to do an arc with just the custom group when I get time. And going by the feedback I have gotten so far, I will tweek this arc a bit.
YMMV---IMO
Ice Ember
Late 30s and large team, huh? In that case, I recommend my "Hunting the Dark Dragon" (2922). It's fairly average solo and on low difficutly, but ramp up team size and challenge rating, and the arc will oblige - though it's mostly the last mission. Still, some people have told me the 3rd and 4th pack a punch too. 
To a lesser extent, this also applies to my "Revenge of Hro'Dtohz" (1356) - easy to solo, but when played with a large team, the Rikti start bringing out the heavy artillery. Not quite as hard since standard enemies are used, but I like to think that the combination of buffs and tough bosses does make it a challenge, especially on the higher difficulties.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
Way of the shinobi arc. Nuff said.
Bump and Grind Bane/SoA
Kenja No Ishi Earth/Empathy Controller
Legendary Sannin Ninja/Pain Mastermind
Entoxicated Ninja/PSN Mastermind
Ninja Ryukenden Kat/WP Scrapper
Hellish Thoughts Fire/PSI Dominator
Thank You Devs for Merits!!!!
It's probably worth noting that we don't qualify as a large team - we have a lot of characters between us, but there are only four of us. We most certainly do run things on high difficulty settings, though
Also I am glad someone is finally wanted to do the harder stuff. Most everyone here complains stuff is too hard and want you to wussify your missions all because they scared of a little build up and aim and minions having mez protection.
Bump and Grind Bane/SoA
Kenja No Ishi Earth/Empathy Controller
Legendary Sannin Ninja/Pain Mastermind
Entoxicated Ninja/PSN Mastermind
Ninja Ryukenden Kat/WP Scrapper
Hellish Thoughts Fire/PSI Dominator
Thank You Devs for Merits!!!!
Oh boy this sounds like a great opportunity to introduce my first arc:
Leapin' Lepretrons #131775
During my oh so robust 1 man testing I found a few things that I thought were too difficult for the general public, so I toned them down a bit. I would LOVE a second opinion.
Three of the five comments (and one full detailed review) I received on my arc to date, state I should 'tone down' my custom group They all stated they were able to finish but that the custom group was overly powerful and a bit more of a challenge then they wanted. I've been debating on how or if I should tone down the group as the other two who commented, loved fighting my custom Villian group. I didn't do any large group testing, so if your group is willing; I'd love some feedback from a group.
With that said:
Title: Is it Live or is it Memory-X
-------------------------------------
Story Arc ID: 70210
Authors Global Chat Handle: @Armsman
Length: Very Long (5 missions)
Alignment: Heroic
Story Description: The 5th Column is building a device to control the world. Can you save the world from TOTAL DOMINATION?! (Hey it worked to get my Villians and Vigilantes pen and paper RPG group interested back in the day.)
Designer Notes: The Arc can actually be soloed by most Scrapper (Level 50 MA/SR), Tank (Level 41 SS/INV) and MasterMind (Level 39 Robot/Traps) builds. My level 41 Gravity/Force Field Controller made it through mission 3 solo with no issues, but starting getting defeats in mission 4; so if youre a support class or a blaster, you probably need a team-mate. This arc is actually based on an adventure I ran players through in the Villians & Vigilantes pen and paper RPG a number of years back. I was surprised at how much of the original flavor I was able to keep in adapting it to fit in the CoX universe. There was only really one element I couldnt include due to MA limitations.
try searching for the challenge arcs that have been posted. there are a few very nasty ev/av combos intentionally put together to slaughter most teams that encounter them. I encountered an extreme level ninja mastermind/willpower eb in a spawn with an extreme fire/storm troller. and the minions were freakin illusion/plant.
we spent half the mission confused and the other half either placated or being attacked by invisible enemies. we had to get a couple of us to drop our damage for some buffs to counter them.
Villains & Vigilantes was what got me into superhero gaming in my teen years. My main character (featured in the FREEM! arc) found its origin there.
As a matter of fact, the FR.E.E.M. acronym I use in this more challenging arc also was invented 20 years ago. 
It can be soloed by most mélée fighters, but I had trouble getting through the AV at the end (purpled out inv/em). I toned him back down to EB level after almost every comment received mentioned it.
Good luck.
█ Players Guide to the Cities
The Chapter and Verse arc in my sig is challenging. It is villainous, though, not sure if that matters to you.
A couple of missions in you hit a few EB set to extreme that make for a nice challenge. I and others have run it solo (fun but painful), and the largest team I ran it with in testing was 5.
City of Heroes was my first MMO, & my favorite computer game.
R.I.P.
Chyll - Bydand - Violynce - Enyrgos - Rylle - Nephryte - Solyd - Fettyr - Hyposhock - Styrling - Beryllos - Rosyc
Horryd - Myriam - Dysquiet - Ghyr
Vanysh - Eldrytch
Inflyct - Mysron - Orphyn - Dysmay - Reapyr - - Wyldeman - Hydeous
My teammates and I created an all defender team which steamrolled missions from level 1-50... we too were looking for a challenge.
Thus was born the Def Enders Competency Test. ID:17966
Loads of debuffs to overcome our capped defense, plenty of defense to combat our capped damage (we had two Kin defenders and double ice shields) and plenty of tough bosses. I've never been able to fight the AV... so I can't comment on her... (she is not NIN if your wondering)
The all defender team consistently wiped when more than one group was pulled. No team I have been able to assemble from 35-45 has been able to get past the main hall.
The mission is NOT impossible... I didn't do cheesy s**t like spamming confuse, TA, NIN. But there are a few suprises which I will leave to you to find...
All I ask is you give me honest feedback about the mish
...Sent you a PM, Svlad.
.
Check... and Mate (poster) - Arc ID# 15095 (comments)
Invasion on Earth BX1132! (poster 1) - (poster 2) - Arc ID# 98943 (comments)
Global @ARH
http://img.photobucket.com/albums/07...ureBanner2.jpg
I made the final boss in my arc an EB instead of AV so it is soloable but I think my entire arc is still a nice challenge. Not aggravating but a challenge.
Its a custom villains group and there are 2 EBs in the arc.
You are most welcomed to give it a whirl
Arc #: 17523 - A Little RnR
Char Site | My DeviantArt
Global=@Thornster
If you are still looking for a challenging Arc.. try:
Terrors of Old
Arc ID: 124746
It has a few Circle of Thorns sprinkled in with a bunch of custom enemies. 4 mission Arc that can be done solo but scales up really well against a party. I think you will have fun with it.
Arc ID 19017
Operation: World War III
I still think it is a bit longer than needs be (minorly tedious at times). In mission 4 ditch the allies as well, they are too strong, IMO.
Why Blasters? Empathy Sucks.
So, you want to be Mental?
What the hell? Let's buff defenders.
Tactics are for those who do not have a big enough hammer. Wisdom is knowing how big your hammer is.
A good two story challenge arc is my wife's, called "Knight of the Mystic Realm." I don't know the arc id# off the top of my head, but you can find it by her global of @Nightshimmers.
Arthurian story line, Custom Critters wielding all kinds of weapons and shields Morgana is an EB you won't soon forget.
It's the second mission that's epic, though. One person going through it on a team described it as "Hamburger Hill" and another described it as "Lord of the Rings" meets King Arthur. There's Bres, Tuatha, Knights, an AV (Mordred, of course), allies, and boss spawns that overlap with rather large patrols. And giant floaty eyes!
It's not for the faint of heart, but it's a ride and a half!
The Scrappers' Guide to Dark Melee | Kheldian Binds and Strategies
Depending on your team composition Three Stars and through the Looking glass can be easy, medium, or "oh god make it stop!" just looked for my global and give up a whirl.
AE # 67087: Journey through the Looking Glass - Save the World
LLX VirtueVerse! - Check out my crazy Toons
This is the size of group that we have balanced AVs for, 6.
-Positron 06/07/06 07:27 PM
I'd humbly point you to my 3 ARC trilogy. Red Dawn has a tough opening mission and both Red Scare and Red Doom have challenging climaxes.
Wow, when the thread wandered down to page 3 I figured it was dead. We had a lot of fun with the first couple arcs from this thread, and I hope to make this my main activity in the game from here on out.
Keep them coming! We will eventually play through all the arcs here, and we will be more than happy to honor feedback requests.
Incidentally, we play on Infinity. If you've read this far you already know exactly what our idea of fun is. If you think the same way, send me a message; my global chat handle is Svlad, oddly enough. Thanks to the Rikti War Zone it doesn't really matter to me which side of the fence your character is on.
I do not believe my arc is the uberest challenge ever (see sig), but I would be most appreciative if your group tried it out and gave me some feedback as to how tough/easy it felt.
The standard enemy is Malta, so if you're playing it in the late 30s and not 40+, this may add on to the challenge factor slightly. Go for unyielding difficulty to raise the spawn sizes. Feel free to ditch the completely-optional NPC allies in mission 3 and 4.
The first mission and last should be the trickiest. The first because of high extra aggro possibility due to map layout, and the last should have a fairly fun end boss fight.
Thanks!
Invictus Est Level 50 Invul/Fire Tank
Malentis Level 50 Ice/Energy/Leviathan Dom (Freedom)
Black Jeremiah Level 50 Fire/Fire/Mu Dom
Sejanna Level 50 Dark/Dark/Elec Def (Virtue)
Arc #119664 - The MiniMech Cometh - Hess TF Mini-Sequel
just wait untill the patch on test gets put on live. then evey singl custom enemy group will be team wiping even the best group gathered to take them on. that patch is soooo broken it aint even funny.
"Rise of the Syndicate" in my sig is quite a challenge. Soloable at appropriate levels. There are AV's that scale down to Elite Bosses so plan accordingly (12 total over 5 mishes)
"They Come from Uranus!" while silly can prove a challenge solo, the mobs are custom but not out of this world (thats a pun =P ). It's only two missions long and contains two EBosses.
I also have a challenge one mission arc that I've been testing out. #130958 'The Black Plague Challenge" haven't finalized it yet though, so I haven't listed it in my sig. Basically tried bending the mechanics of the game a bit to create "swarms" of my custom ratmen.....would love some feedback on that one.
WARNING: All story arcs actually contain a plot!
Try 95278 - Gathering the Four Winds; it is the toned down version but is getting ramped up as my SS/WP Brute gets better at it.
Mission 1 has 3 Bosses and 3 Boss Ambushes
Mission 2 has 4 bosses and the Lt are Standard Empaths
Mission 3 is an MM fest
Apparently, I play "City of Shakespeare"
*Arc #95278-Gathering the Four Winds -3 step arc; challenging - 5 Ratings/3 Stars (still working out the kinks)
*Arc #177826-Lights, Camera, Scream! - 3 step arc, camp horror; try out in 1st person POV - 35 Ratings/4 Stars
You can try my Arc called Echoes of Grackus. I don't have the arc ids right now, but you can search the word Grackus and find them (pretty sure I'm the only one that has a villain named that). One EB in the second mission uses Inferno which wipes everyone if they are in the same room! Also in the last mission there is a DB/NIN stalker that two shots my tank lol!
Well, having pretty much gotten my message across in the thread title, this is going to be a very exciting post. :P
Ahem. More seriously, in the absence of an arc tagging system or somesuch, I need your help. I and a couple friends came back to City of.. in the hope that we might find that the Mission Architect would provide us with new challenges. I've tried searching based on the current tags, I've tried asking on global chat channels, and so far all the missions we've tried have been steamrolled. Reading other threads, I know the fire blast/willpower AVs are out there. Help me find them! Send us to your worst nightmares.
Should it be useful, I'll provide some additional details. We've got a huge variety of archetypes and levels on both sides of the fence between us, but the major intersection tends to be a group of heroes in the late 30s. We've got a lot of strong control, debuff, and pure damage characters in that range. I know we're nowhere near invincible.. we're just not finding the deadly missions. Help us out, please.