Tactics against Brontors...
 

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?

Ball of Lightning for the win :)

Max your pet Dragon's Ball of lightning ability and get some Gorguanas. Mark of Blood on the Brontors and create the ball of lightning somewhere close to them. Now watch the carnage! It is possible to kill 96% of that stack in a hit and inflict a wicked "damage" (it's not damage, it's how much HP the Ball of Lightning killed) - you can refer to my signature for additional info... I wasn't bragging, I simply put it there as an example of a very, very useful trick in the game.

Currently I got 21 rage and since I'm only using 2 stacks of dragons in most of my battles (Mage currently lvl 25-pet dragon 17) Balls of lightning doesn't seem like a viable option for me. The question is still open: any idea how to deal with them?

The boring way... level 3 target + stone skin...

You are unlikely to get any Gorguanas outside of Reha so you must do map kiting to enjoy the 96% kill.

Burn and poison them, it can wear away a lot of the stack.

I agree with lauvhk, the easiest way to deal with them is to cast Target Spell lvl 3 and Stone Skin Spell on one of your dragon stacks. Then i would probably use Defenceless Spell and Ghost Blade on the Brontors to kill them.

Phantom or Demon Portal if you have it.

You don't need any army actually, use 1 archdemon for his high initiative, cast invisibility on him and kill them with spells . (u can use more than 1 if u plan to do damage with them )

Actually you can do the whole game like this, (for enemy heroes use 1 or more black dragons and thats it ) For spells use geiser, fire rain etc . i found Geiser to be the most op spell in the game , at my game end it did 11k dmg max to 8 units stacks (including gremlin towers and bosses ) .

@GodClone
Geysir is 40 mana. How do you get enough to kill an enemy by Geysir? You would often need 5 or so? My Geysir at level 42 gives me about 3200 damage on a unit.
Are you Kiting a lot (I dont kite or reload)

My geiser does over 11k max dmg . so i geiser all (unless its less than 4 stacks, then its not worth it, just fire rain ) . Got 160~ mana depending on what items im using, on boss fights i have 120 b/c i stack resistance .

Just use calm rage when you run out of mana, 1 mana for turning x rage into y mana . Ez . I can use up to 5~ geisers per battle, thats around 50k dmg / stack . I flame arrow big stacks for burn dmg and geiser on same turn, since i can cast 2 spells /turn .

Kiting is boring and tedious , i don't do it usualy . Did it once when i was lvl 20 in verona when fighting the castle guard the guy with huge army, i had 1 black dragon :))) .

Oh, those are some of my end game stats, if you are talking about early game, geiser is not worth it, just fire rain .