How do you fight Death Underwater?

[Soyweiser]

Use D random mages to cast wither bones. Cast summon water elemental. (and hopefully you took Cold dominion and get Ice elementals).

Use mind burn to kill the commanders, mind hunt (ritual) to kill the commanders.

Cleansing water spam works vs undead. (oops but not underwater).

A well, commune some mages together to have a few of the D random ones cast wither bones, or just use the astral booster to raise their D3+ level.