Quick look, and first point is that I've gathered noone really enjoys /ninj' foes. It's "usually" an instant no no. However you are making a challenge, so go for it!
AV looks fun. 
Fury
any ideas for powersets on minions? ones that will make them real pests
Anything that slows, heals, blinds, buffs or debuffs.
Awesomely difficult for everyone or just for non-melee archetypes? Giving control powersets to minions, and giving them an armour set with status protection so that they can't be controlled, is really going to hack off dominators/controllers/kheldians, certain defenders/corruptors, might annoy blasters/masterminds in certain cases, and probably won't be noticed by brutes/stalkers/scrappers/tankers/VEATs at all. You need to know your target audience because awesomely difficult for some archetype/powerset combinations might be insta-death every time for others, while being a walk in the park for another build.
Force-multiplier powersets on minions like Kinetics, and to a lesser extent Dark Miasma and Radiation Infection, are really going to hack off large teams because the more minions there are the more debuffs the alpha-takers get hit with, but soloers won't be affected so much.
Putting PBAE heals on minions will make the spawns tougher the larger they get - with potentially the whole spawn being fully healed every few seconds so you'd have to kill everything in 1-2 hits.
Players generally find lots of Caltrops, Glue Arrow or Quicksand patches annoying so you could make sure multiple minions have those to stack up, to slow players to a crawl. Quicksand is probably the worst as, unlike Caltrops, you can't fly out of it and it has a huge -def effect that really hits all Defence-based primaries hard. The -rchg on Glue is no fun either though when stacked up - a couple of glue patches and your attack chain can slow to one attack every 5-10 seconds. These patches would annoy melee characters more than ranged though.
But you need to understand how all the powersets you put into your custom faction might be used in concert with each other. One of my global friends made an arc with custom characters with Ice Armour, but also had some mobs that buffed them with /Cold buffs for even more +def - that made minions/lieuts very tough but also very annoying as fights were taking ages to finish as you just couldn't hit those buffed mobs until the streakbreaker forced a hit.
As for cheap and undefeatable, I'd avoid hard hitting melee sets like Super Strength and Energy Melee, especially if the mobs have the Rage/BuildUp powers as well - combined with autohit damage like Caltrops keeping players below full health, the one-shot code won't save players from instant death if they're hit for more than their maximum health (which is likely against SS or EM).
Alright, i'll consider that
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any ideas for powersets on minions? ones that will make them real pests
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*/Cold Dom Extreme minions will make most things weep. They get Infrigidate and Benumb (so long Defense, Recharge, Regeneration, Damage and Run Speed and have a negitive Power Boost applied to all your mezzes, some buffs and debuffs and your heals as well), or at least they did in Test.
In test I made some Dark Blasting / Cold Domination minions who did awful, awful things to my Brute. Pairing them with Plant Lieuts didn't help matters much.
I need suggestions for AWESOMELY difficult mobs. At 1st i tried elec/dark miasma, but this made my minions near impossible to beat.
I'm looking for them to be as HARD as possible, WITHOUT being completely cheap and undefeatable.
I've experimented with elec melee/psionic assault also.
Suggestions on ownage minion setups will be appreciated.
My LT's are:
Elec melee/ninjitsu stalker
Elec melee/radiation
NRG melee /pain domination, again, improvements are welcome!
Bosses are:
Katana meleee/ice armor
Elec blast/dark miasma
Ebs are:
Mind control/kinetics ranged combatant
Broadsword melee/fire manip
AVs:
Mind control/mental manipulation
NRG melee/regen
Please criticise my choices and tell me if something is good to keep, or if i could make anything more difficult.
I really want to improve on the LTs and Minions though. Remember i'm looking for them to be painfully difficult, i WANT people to die multiple times and have to come back to finish.
Thanks in advance