Tactics against Brontors...

[tofucsek]

Playing on impossible (aiming for no loss playthrough) I'm having a problem with these guys. Being a mage, I don't have massive army to kill them and once they've used their burrow ability I'm pretty much screwed. As they are immune to mind spells I suppose I can only kill them with direct damage spells, however even in Montero I used to see about 120 in just one group and I hardly can imagine any spell that would deal with this ammount. Question is: how to beat these ones without any of my dragons dying?