Trojans
http://wiki.spiralknights.com/Trojan
may be invincible when you fight them one on one (if you play solo or if your party is busy/dead). They just charge you into a corner and you can not:
- get out of the corner
- move trojan - they attack you too often to charge a weapon
- kill trojan - their shield blocks ANY kind of attack and is invincible
The best part is this: after you die, trojan may just stand there! So even if you do spend energy on resurrection, you still won't be able to move or do anything! Basically wasting all energy spent on respawning. This goes in contrast with level design, where you can always progress without getting yourself killed.
Simple solution: breakable shield. If it takes too much damage, it breaks and
a) trojan retreats to recover shield, just like knights
b) trojan becomes vulnerable for a period of time, so he can be killed
c) trojan stops attacking for a period of time, allowing one charged attack to be executed.
Not getting yourself into a corner is not a solution, thanks to lags trojan's attacks are nearly impossible to predict - they just throw you around the map randomly, at some point you suddenly appear in a corner with the big invincible horse mashing your shield.
Sorry to burst your bubble on this one but I solo almost 100% of the time so I've fought and killed a lot of Trojans. Yes, the "holding you in the corner" thing can get annoying, but you can usually solve it by using your shield at the appropriate time, having a weapon with serious knockback or, if all else fails, waiting until he moves away after you've died, then rezzing.
Soloing a Trojan takes a different strategy than in a group and mostly seems to involve a lot of practice in timing. Keep at it and I'm sure you'll get the hang of it!