Spellcraft 10 is enough, to wear the orbs and staffs. More is not needed and better invested in the power of your spells.
Resistances are nice, but normally you can avoid being hitten.
I think resistances stack, but you wont need that (even on hard, which isnt so hard ...).
Ranged combat is a good thing, but two tanks are more important. But hey: both ways are ok. And the most fun is 2 mage/2tanks (probably one evade-tank) imo.
With a little bit grinding in level 6 (firemaze and skeletons/snails going from one to the other) you can reach about level 15 without problems.
And i'm sure, that you will master the game without problems with 4 mages, if they all have enough in ice (13) to cast frostbolt.
The only mobs which resist ice are ice lizards in level 9, and then you have enough power in fire and air to blow them away
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Invisibilty works as intended, but is resolved if you are on a square next to a mob. Darkness is only for instantly lights out, not really needed.
Both are necessary to enter the fighters challenge for reaching a nice spot to fight (behind the gate, using golden key)