Ice snake also deals physical damage, so it's all the same. Geyser can hit up to 8 targets with level 3 distortion, and it puts ice snake to shame. I personally doubt that all those targets will have more than 10% physical resistance. The best is 30% anyway, at least for enemy stacks.
What's good about death star is that its damage cannot be mitigated, since it's astral. Stock up heavy on intellect items and try it again. Death star is the best thing to use against dragons and the best things to use in battles where there are lots of enemy stacks (the battle with the Champion in Verona is such an example), as they are bound to line up properly for you.
This is why you need to have a good spell arsenal. Against very big stacks of high phys resistant units, use Flaming arrow and poison skull, as they will dwindle them down due to their post-effects. Against lots of stacks, use geyser. Against lots of stacks with good resistances, use death star. Keep a few defensive spells for your own troops and that would very much mean GG
For the last battle, get eviln, get black knights and baal will go down easily. No need for offensive spells there.