Wind Guide, Touch of Madness, there are probably others.
Many of the best mass troop buffs (offensive or defensive) require gems. Perhaps if the AI was better at judging when to use gems for these spells it might be better.
yeah well I forgot about some offensive buffs, but which would you rather cast:
fog warriors, will of the fates, mass regeneration and army of lead, or all the offensive buffs mentioned?
I have recently been using Mass Flight, which is awesome for storming castles with a large army. However, the gem use problem is there. Often another battle in the same province in the same turn will drain the caster's gem box before the castle storming.