[Gatts]

In general "become available later in combat" is valid for mage (and little bit for paladin)...
Mage has penalty for rage increase
Mage has the least powerfull armies, so it is very rare you kill whole enemy stack with first hit (as warrior I was able to kill always one enemy stack with archers/ skeleton archers... as archers are low level units, and the killed unit was higher level, I got 60+ rage with one hit only... )
Mage has not only lower Leadership + rage increase penalty, but also lower Attack... this three elements in mutual conjuction end in fact, that you get +1,+2 rage per hit... there are only max 5 hits (five army slots)... so if you don't trhow strong damage spells/ or two spells on your enemy, your rage per turn is 10~20 max (if I count in luck of critical hits, or good strategy, without it would be 5~10)... but well... you can shoot 2 maxed fireballs on your enemies, and get 10 rage from each (even more if you kill a whole stack)... depends

Fire spirit is good IMHO - but he fits much more warrior/paladin, (as spirits should balance the game against spell-strong mage) underground spikes, when maxed only for 40 rage hit everyghing on map what is not yours and deals 1000~1500 DMG (if target is mage-like then 2x so much), also the wall could help a lot to save at least one turn between your archers and enemy tanks

Charges were for me very useful, mostly for regaining mana as warrior, or for regaining rage as mage...(still it may be more expensive, then give rage... but with inquisitors, it is working OK)

Sleem is great, if you are strong mage, you can use gloth's armor together with armageddon... if one armageddon is not enough, use armageddon , ressurect, armomr and aramageddon again...