Personal experience, but I found that something like Katana (or any weapon melee powerset)/Radiation Emission works quite well together for a Boss character. The constant recasting of the Radiation powers helps to dampen the normally overwhelming damage a custom Boss can put out.
I ran into one arc with a normal distribution of blaster and scrapper types in their custom mobs - and a Plant/Empathy Lt that was just absolutely the most annoying thing in the world if you didn't pound it into the ground right away.
Damnable Carrion Creepers!
@Mindshadow
I have found tar patch in dark miasma is a fight changer. The heroes get stuck in it and lack movement. Add minions with ranged attacks and it becomes a death trap for the hero. But a hero who thinks will move out of the tar patch - it can't be recast. So it both limits movement and encourages movement.
Any healing is fight changing. But if minions heal there is nothing the hero can do - it is just a pain. Lt's or bosses healing makes it a tactical situation - take out the healer first.
I second the Kat/Rad Lut suggestion. I also found that giving Invulnerability to a boss makes them incredibly tough even at the hard setting. Extreme is almost unkillable. WP bosses on hard are tough, on Extreme are annoying.
A PSI assault / WP boss is a fight changer... you had better have a Kin or Dark defender heal ready when that PSW hits because it's going to knock half your team's health off in an instant. I have a Psi/WP boss and a MA/Sonic extreme boss in one of my maps... when the Sonic throws out is liquify and the PSI hits PSW... it's over.
I give heal sets to lieutenants and above, and give tanking defensive sets, especially the ones that include raw +HP as their base powers, to minions. Adding +HP makes it slightly more likely that your mobs will survive a hero alpha strike. The lieutenants probably will, and if they have heals they at least get the opportunity to heal the surviving minions. At certain difficulty settings, some of the healers can resurrect minions.
The healing sets differ in effectiveness. Empathy is not as tough as Pain Domination; and Thermal is the most difficult for players to deal with. Pain and Thermal are better handled by the AI than Empathy is, IMO. On high settings Thermal has nasty debuffs.
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Plant control suuuuucks to go up against. I learn fast which mobs have that set and they go to the absolute top of the kill list.
I'm trying to balance for solo rather than teams, on the assumption that most people running arcs are going to be solo/duos rather than teams.
Altoholism isn't a problem, its a calling.
60+ characters, 5 years, 3 50's.
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I'm talking about powerset and unit mixes that just worked well together. Interesting and tactical problems without instant lose situatuions are the goal.
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When creating custom mobs, my goal is to create multiple, interdependant threats. If you take out the healing mob, the damage mob is free to pummel you. If the damage mob is attacked first, the healer will be free to heal. I also segregate the threats based on mob level (minion, lt, boss). My goal is to reward those who take the time to analyze the threats, and punish those who run in and mash buttons. My arc, "Collecting the Evil Doctor's Debt" is a good example of this.
Maybe the biggest challenge is how mob spawn rate affects threat level. What I find to be key is creating different versions of mobs in which the more challenging power is replaced with one less threatening; while the secondary remains the same.
The only real problem I am having so far is in trying to create a custom EB which doesn't have the ability to drop players on a bad roll.
I think the Devs are datamining everything we do in AE and how people fight AE-customized mobs so they can then tweak the AI scripts of the in-game regular mobs to make the game more challenging... imagine MindControl/DarkMiasma Minions surrounding Dr. Aeon when you come to fight him... Mu-Ha-Ha-Ha! *cough!* 
I believe that a Kheldian Gold Standard should be based on SO's, and for anything above that... there's Platinum!
Save Ms. Liberty (#5349) � Augmenting Peacebringers � The Umbra Illuminati
Save Ms. Liberty (#5349) � Augmenting Peacebringers � The Umbra Illuminati
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Maybe the biggest challenge is how mob spawn rate affects threat level. What I find to be key is creating different versions of mobs in which the more challenging power is replaced with one less threatening; while the secondary remains the same.
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I'm not sure I'm appreciating the meaning of this. I think I know what you mean... but perhaps not.
By mob spawn rates: I'm currently toying with eliminating minions all together. I've got some very hard hitting mobs (SS (ex)/Electric manipulation (ex)) in order to provide something that really needs to be controlled or killed quickly even if there's a granite on the team... and some less hard hitting mobs: BS (wk)/WP(ex) and Psi assault(st)/WP(ex) to tone things down, but to give a "controlled" spawn some staying power. I may have to add some FM (wk)/? mobs as damage dealers as well to further limit things. This is in addition to the more dedicated ranged support that could spawn.
If you are suggesting that total spawn threat can be managed by diluting the pool by an appropriate degree, then I get what you're saying... If you're saying something else... er might I ask what that would be?
The cake is a lie! The cake is a lie!
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When creating custom mobs, my goal is to create multiple, interdependant threats. If you take out the healing mob, the damage mob is free to pummel you. If the damage mob is attacked first, the healer will be free to heal.
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I find those are always the most interesting groups to fight. Consider Malta - every archetype has a specific enemy they beeline for. Squishies hate Spec Ops, as their stun grenades last a ridiculous amount of time. Tanks and Scrappers who aren't Defense-based hate Sappers, since they're susceptible to having all of their endurance drained and their toggles dropped. Defense-based characters hate Gunslingers, who have a high chance to hit that no amount of Defense can negate.
Including a potential Kryptonite for everyone into your enemy groups can make for a fun group, as long as you don't go overboard.
We'll always have Paragon.
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If you are suggesting that total spawn threat can be managed by diluting the pool by an appropriate degree, then I get what you're saying... If you're saying something else... er might I ask what that would be?
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I think you got most of it. In each spawn, I want a certain amount of a certain type of threat present. To do this, I increase the number of mobs in that group.
For example:
I notice that each spawned group tends to have two mobs with radiation infection, but one would be a much better challenge (stacked Rad infection gets way too hard). To lower that number, I create another mob to add to that group level. This is a perfect spot to create a new threat which may be no big deal to those who get pwned by rad infection, but it is a problem for those who it isn't. Thinking that, I create a mob with storm summoning to introduce some knockback. Variety is key.
You can also create weaker versions as I did when I decided that I wanted lts to have elec blasts, but multiple versions of volt sent would be too much end drain.
Determining the threat of a custom group is a huge balancing act. One that I am still very much learning. 
I think the most memorable mobs for me are those that have become the biggest p.i.t.as for me. A friend did a demon mission when they were essentially all regens and so rezzed upon dying. And the first mission was a clear all! ACK! lol He just made another arc last night with probably the hardest mobs I've ever experienced (on rugged no less. And if you knew me, you'd know I scoff at anything under invincible). But the sonic/SR bosses were brutal!!
I also had an issue with having a /ta minion in my arc. Having triple stacked flash arrow wasn't much fun. In the end, I ended up promoted it to LT so it wouldn't spawn as much and have more powers to use.
Arc 185502: Who Killed Snow Globe? a mini mystery Put together the clues to solve the case!
Arc 22832: And Hell Hath No Fury (extreme)
Will you be the key to the Knives of Artemis' survival? or the instrument of their destruction?
I've been playing with this one arc for a long time now... since well before live, but I just can't seem to get my mobs to work together the way I want... Oh, Im inching towards something acceptable, and some individual mobs are exactly where I want them... the reasonably delicate but terrifying glass cannon, the durable as hell but not that dangerous mob... but they don't all "work"... or if I make that unwise change, they end up working "far too well", especially at a harder difficulty level.
I'm curious to know what custom mobs people have come across that they really loved.
I'm not talking about costumes and concept. Those are critical too, but that's a different discussion.
I'm talking about powerset and unit mixes that just worked well together. Interesting and tactical problems without instant lose situatuions are the goal.
Your foes were hard if you charged in without thinking? they were easy if your team did it's job? That's exactly what I'm looking for.
Any references would be much appreciated!
The cake is a lie! The cake is a lie!