For Firkraag, instant death spells. You can kill the shadow dragon normally though, just casat every protective spell you can think of, lay a dozen traps, then hack away...

Yoshimo will betray you no matter what. I do believe there is an exploit however...

As for the perfect party, well, I usually used the Drow cleric(if I'm weak in melee, I sometimes use anomen instead), Jan, the blade, the paladin or mazzy and Edwin.

The underwater city is very good and fun, so you'll want to get there.

When you have killed enough of those elementals(it takes a lot), then they shut down and something I don't remember happens.