You can either not use area spells, or choose not to fight them in that place. All you have to do is go a few rooms away, and their mob summoning just wasted a lot of mana. Summoning also has a sizable lag, so those non-area spells are effective. If all else fails, leave the area. Welcome to the more defensive life of goods.
Although, I can't imagine what class you are having a problem as. Invokers do not -need- area spells against those classes that can summon (except maybe decked clerics), and bmgs are hardly bound by flashfire. Ninja can push and have fired weapons, and rangers have fired weapons. Bard's season is dangerous anyways. What else?
My suggestion is to use some of the 10000 consumables available to goodies to more than counteract this basic strategy.
Edit: Good cleric's earthquake? Doubtful though.