[tinkthank]

I'd not recommend archer types quite so much since you cant shoot underwater.
I have no experience with Arco GA or what your pretender is, but you get lots of elemental with astral, right? Air and Water will get you the items that take you underwater; then I'd go mounted troops to reach his rearmost fast.
Do you have any death mages? Lots of undead work well too, since they are mindless and hence immune to mind bLast; magic resistance on your mages helps too. Your astral mages can mind bLast as well. I think it's a level 5 conjuration spell, gets you a dozen sea trolls and sea troll capnt, forget name, that should do the trick too. Once you get one prov conquered, start recruiting tritons.
Is that helpful?
What paths you got researched? How far into the game are you? What's your pretender?