Advice for Mages (and paladins).
First - when you get Rage upgrades, try to choose the ones that do not involve huge increases in Rage. The Ice Ball is a very usefull unit even on low levels, when it only costs 10 Rage. Same with Glot armor - it will always absorb 5000 or so damage, regardless whether it has 400 or 700 HP... so try not to upgrade it that much.
Second: get items that increase rage generation. Spear of Rage is often avaliable in the Creiston Mines after you beat Baron Ahey, or elsewhere. However, you are even better off if you get the Pain Skull. It's more usefull if you get it early on (I was lucky, a ghost stack in the Marshan Swamp had it)... if you manage to fight enough battles, the skull will not halve your Mana before combat, and it gives you +50% Rage increase. Meaning, that usually for a Mage you'd get 1 points of rage when you kill a full stack - now you get +2. And you can get huge amounts of Rage this way when using damaging area spells, Lightning can net you +10 or so Rage even! I find it much more usefull then the Dead Skull (it gives +5 Attack and Intellect- I have about 4 items that give me more then this amount AND provide better bonuses like resistances). It's hard work getting the skull up to this point, though. I recommend you do easy battles with it, for example the pirates in Freedom Islands, or get an easy to upgrade item and fight the keepers, then get the morale low and fight it again.
Also - try to kill full stacks (or have your own full stacks killed!) to accumulate more rage. This works if you or the enemy have units that can summon/spawn troops: Gremlin Towers, Druids, Dryads, Royal Thorns, Necros, Demons. It's a well-used strategy of mine in keeper battles to leave a "friendly" gremlin tower intact, then one-hit kill the troops it summons (friendly gremlins can only cast slow as a debuff/attack spell so they cannot hurt your units) to get instant rage. Or use Reaper's Rage Drain on them.
Also increase maximum Rage. There are Rage skills that you cannot even use with less then 30-40 Rage. Plus the more Rage, the more chance of criticals. There is a might skill for this, as well as items, and of course, roasted rats. They taste 'yuck' but they are good for you!
Zerock is probably the least usefull spirit - in later battles his damage won't be big enough, and his abilities are of little tactical importance. The wall should at least provide cover from enemy archers to be usefull - and it has a very limited lifespan. He's best used against Mage units (Necros, Priests, Mages, Shamans) because he does extra damage againts them.