What lv are you fighting these elite boss's in villains, could make all the difference since atta and frosty are both in the low lvs and so can be a real pain to fight even though nowadays i can solo both of em 
since I6 came out. Yeah and those Balista's can be a pain in the butt to beat
...unless you're a Necro/Dark Mastermind, then they're pushovers. 
Can't remember what level I was when I fought my first EB but haven't had a problem really with any of them & am on one from top diff. Got Barracuda today so might change my opinion yet but I doubt it.
Yeah bet it is easy for a Mastermind but then I can do it without all my mates helping me
The Good:
Krazy Mary L50 Kinetic defender
Uniqua L31 PeaceBringer
Black Annis L24 Warshade
Krystal L18 Ice Blaster
The Bad:
Frenzi L34 SS/Inv Brute
After not fighting any EB for a while I recently came across the expidition commander ( the cov shadow shard mission ),ive got to say,he was quite tough,I remember I basically solo'd every EB,in cases where it got tough I had a shivan tank the boss while I pounded him,but this guy was extremely tough,first EB that pose a thread to my brute.
well it all depends if its a "proper" EB, or a downgraded AV. Proper EBs most of the times aint a problem for a brute, unless they are ballistas . A downgraded AV (which barracuda most likely will be) can still be a major pain in the butt.
Oh, dependin on what kind of brute you are, aurora borealis and infernal can be a pain to solo as well at the later lvls
Btw, the reason why frosty and atta are hard is cause of: you are quite low lvl (frosty usually at lvl 10 or so), most likely arent properly enhanced, you can be a 'pure' support type, frosty and atta are a wheee bit overpowered for their lvl (curse frosty and havin both fire imps and jack frost, I want that at lvl 10! 
) etc etc. Still can solo em both with relative ease, just takes some thinking
"god, how many devs did hami have to sleep with to get ED?"
Total Cat @Stagefright
Only had one give me any problems and im on the on the second from toughest difficulty, it was the 45+ snake EB, thats because it liked its toxic damage and a fair bit of DoT and me being energy diddnt 
. plus the first time i sprinted into the room and about 10 bosses hatched 
. Had to do it on heroic (still spawned as bosses) and take it slow. balistas are no problem i took 2 at once with a few ins 
, Not boasting could have just been luck and overload
Okay I have got some respect for EB's back, Barracuda handed me my perfectly formed butt a couple of times & I had to call for help (thanx Woman of war) but thats cos she was a psionic fish thingy & I have zero defense. I look forward to all the other EB's to come :=)
The Good:
Krazy Mary L50 Kinetic defender
Uniqua L31 PeaceBringer
Black Annis L24 Warshade
Krystal L18 Ice Blaster
The Bad:
Frenzi L34 SS/Inv Brute
was kinda put of playing my brute after collecting a fair bit of debt from AV missions after the realease of i7, and thought last night when i got the eb miss, it would be the same. How wrong, ballista, barracuda and one other eb that i cant remember, all went down with ease. Yeah i had to use unstoppable as my end was a bit low, but i was relatively surprised how easy they were, and that was +2 lvl.
Most downgraded AV's I've fought have been easier than actual elite bosses. For example I found Widdershins a lot harder than Citadel.
course all depends on the AT, for instance I always found ballistas pussies the day I met em on my MM, yet citadel is a whole different league to him
"god, how many devs did hami have to sleep with to get ED?"
Total Cat @Stagefright
Just solo'd that crazy scientist from cov,forgot his name for a sec,aoen or something,was an orange EB to me,but took him down pretty easy.
Widdershins is nasty. I general I dont have many problems with EB's on my MM. As far as I can remember the only 2 who have caused me too faceplant in the 40+ region were Widdershins and Pyriss.
I have been mainly soloing with my Brute, set on the one from the top difficulty and bar the Longbow balista who took 2 attempts, all the rest of the EB's have seemed right wimps. I have tried soloing Frostfire & Atta on CoH before & it has usually ended in tears but these CoV EB's seem soft by comparisson. Are they a bit soft or am I just too good?
The Good:
Krazy Mary L50 Kinetic defender
Uniqua L31 PeaceBringer
Black Annis L24 Warshade
Krystal L18 Ice Blaster
The Bad:
Frenzi L34 SS/Inv Brute