
I would guess that if you don't have some pretty high damage output yourself, you're going to founder. I usually double-stun with Plague Doc, front-row stun with Crusader, and then start chewing through their line.

Soaking damage through 30%+ PROT might be a pretty viable tactic, especially if you kit him out with +PROT or +HP trinkets. Actually, if you're having trouble with their relatively high damage output, you might consider bringing a Man-at-Arms and having him shield up.

The spear/sword fishmans aren't really that resilient. If you can do that, then you should be able to manage the rest. Really, you need to kill the fishman casters first, since they heal and buff the rest. I don't know that I'd bring both at once, but they're both pretty useful. Plague Doctor's Blinding Gas is a useful double-back-row stun, Plague Grenade blights back two rows, and she can cure bleed.

Occultist's Abyssal Artillery targets those awful casters and gets +damage vs. My two favorite Cove classes are Plague Doctor and Occultist.
