In no particular order:
<ul type="square">[*]Spectral Daemons[*]Flamethrowers[*]Longbow Ballistae[*]Any 1-shotters[*]Sorcerers[*]Red/Blue Ink men[*]Ruin mages[*]DE swarms[*]Snipers[*]Anything that teleports[*]Toxic gas missile turrets[/list]
Atm the lost swordsmen 1 shotting my poor ninjas 
Suff that makes burn patches! Stoopid robots run all over the place to get away from them, or run nowhere at all!
IN MEMORIUM OF GAYBABY
CRUELY TAKEN FROM THIS WORLD WHILE SO YOUNG
[ QUOTE ]
So i just happened to think about what opponent you hate the most, that cause the most Problems / minion losses etc? even your own life sometimes 
My top all time "oh no" opponents are as follows:
1. Some Tsoo bosses can one-hit my robots. /hardhit
2. Spectral Demon Lords oh man, most minion losses ever.
3. Berserkers that decide to blow themself up, and take my minions with him.
4. More than one guy with a flame thrower, can you hear that? yes it's my minions melting away.
5. Gravedigger bosses caused some losses as well.
6. Ruin Mages, just downright annoying.
The winner of minion kills for me is the Demon Lord. 
Fight goes like this, 3 battledrones dies : resummon, protector dies : resummon and sometimes even 2 more drones dies before it does , argh.
Edit: my build includes poison with the healing but sometimes all the heals in the world isnt enough 
[/ QUOTE ]
Strange, I don't have problems with Spectral Demon Lords. Target them, tell bots to kill, lob web grenade, shoot it, shoot it, lob another web grenade, watch as SDL dies. If necessary, drop Triage beacon any acid mortar near bots to soften up and Spectral Knights that try to pick on my firing squad, problem solved 
TBH, I don't really have problems with many things at the moment, Ruin Mages can be a pain in the backside, but TBH, they're a pain in the backside for everyone.
Arachnos Drone Controllers can be trouble, as theit AoE attacks have wiped all my drones in a couple of shots before now, but that's about it.
Warning:
The above post may contain Cynicism, sarcasm and/or pessimism. If you object to the quantities contained, then tough.
[ QUOTE ]
Suff that makes burn patches! Stoopid robots run all over the place to get away from them, or run nowhere at all!
[/ QUOTE ]
This is true the council and longbow are bad for this with there flame throwers, causing havoc. But on the whole theres no real enemies I shy away from, maybe freakshow tanks as they can one shot my drones and protectors sometimes.
But that lot are easy compared to when i met the evil [censored] Aurora Borealis hero no less, made a right mess of my boys many times. Had to lv up and redo the mission when she went red, but even then took about 10 min solid fighting recalling all my bots at least once and the drones twice. And now I have to face her again in another mission part of one of the story arcs in nerva, oh well.
Aurora Borealis
New least favourite enemy group.
Scrapyarder Demolitioninsts with their ability to drop 5 burn patches in one go. As an aftereffect to the damn great explosion that takes off half your health in one shot
Plus, they don't drop Magic DO's
Warning:
The above post may contain Cynicism, sarcasm and/or pessimism. If you object to the quantities contained, then tough.
[ QUOTE ]
Strange, I don't have problems with Spectral Demon Lords. Target them, tell bots to kill, lob web grenade, shoot it, shoot it, lob another web grenade, watch as SDL dies. If necessary, drop Triage beacon any acid mortar near bots to soften up and Spectral Knights that try to pick on my firing squad, problem solved
[/ QUOTE ]
That is my problem, i'm lacking any croud control abilities but i have found a workaround for my character, just strong debuffs (weaken, noxious breath, envenom) works wonders against Lost LT's too, that can 2 hit battle drones, but now it takes 4 if he can even get close enough. But then there are times when i miss all 3 debuffs 
[ QUOTE ]
That is my problem, i'm lacking any croud control abilities but i have found a workaround for my character, just strong debuffs (weaken, noxious breath, envenom) works wonders against Lost LT's too, that can 2 hit battle drones, but now it takes 4 if he can even get close enough. But then there are times when i miss all 3 debuffs
[/ QUOTE ]
TBH, I don't even bother with the web grenade any more. I just add my own damage output to that of my minions, and by the time they get close, the acid mortar has been softening them up so that the bots rip them apart, even through the acc debuff PBAoE that the SDL's run.
Warning:
The above post may contain Cynicism, sarcasm and/or pessimism. If you object to the quantities contained, then tough.
my bots only seem 2 have a problem with scrapyarders demolishers
Anything that teleports. My pets chase them and aggro the entire map. Then like good little pets run back to me. However like bad little pets they bring the rest of the map with them.
wihtout doubt those totaly annoying demolishers. 1 hits can be easily taken since its only one, but this ******* can take out my entire pet-army if they are all in that stupid burn patch.
Luckely at 28 i finaly get a hold power
50)Sinergy X/(50)Mika.
(50)MaceX/(50)Encore
Sign the petition, dont let CoH go down! SIGN!
[ QUOTE ]
Anything that teleports. My pets chase them and aggro the entire map. Then like good little pets run back to me. However like bad little pets they bring the rest of the map with them.
[/ QUOTE ]
Simple solution. Keep an eye on things, and as soon as you see your pets start haring off, use the predefined Follow me : Passive mode macro. Or pick a new target and tell them to start killing that instead. Or put them in Stay mode : Aggressive around you in preparation for when the porting mob returns...
Sorcerors/Porters always come back eventually, use their absence to mop up other mobs or run repairs on your army
Warning:
The above post may contain Cynicism, sarcasm and/or pessimism. If you object to the quantities contained, then tough.
So i just happened to think about what opponent you hate the most, that cause the most Problems / minion losses etc? even your own life sometimes
My top all time "oh no" opponents are as follows:
1. Some Tsoo bosses can one-hit my robots. /hardhit
2. Spectral Demon Lords oh man, most minion losses ever.
3. Berserkers that decide to blow themself up, and take my minions with him.
4. More than one guy with a flame thrower, can you hear that? yes it's my minions melting away.
5. Gravedigger bosses caused some losses as well.
6. Ruin Mages, just downright annoying.
The winner of minion kills for me is the Demon Lord.
Fight goes like this, 3 battledrones dies : resummon, protector dies : resummon and sometimes even 2 more drones dies before it does , argh.
Edit: my build includes poison with the healing but sometimes all the heals in the world isnt enough