Leveling up Spirits
 

So I'm looking for any advice people can give me on which Spirit abilities to level up from the point of view of a paladin on normal.

I know rage cost decrease is good and rest decrease is good, but at what point do I stop leveling ability X because I don't want the rage cost to go up?

Are there any specific abilities I should focus on leveling?

Are there more than 4 abilities? Do I need to pass on a particular one in order to gain different ones? I'm assuming no but just wanted to make sure.

There each of four rage spirits has only 4 abilities. Well, you can rise, until your max. rage, clearly, cause, if you raise, it's cost more, then you can't use him ;) About spirits abilities leveling more detail:
1.Zerock- suggest underground blades, it packs some dam. early, and later anyway you will more use other spirits.
2.Sleem- glot's armor are very useful, but not worth to lvl-up absorption.
3.Lina- chargers- should max., helps to recover mana, ice thorns- should max., for less cost, they can proptect your units quite well ; for rest- ice orb deals way more damage, if rolled, through field, but gizmo can add, or remove action points, choice of style.
4.Reaper- soul draining , try keep about 40% , if have enough rage, does insane dam. :); time back - should be as high, as your max. lvl unit, for use ; black hole- do not worth, for paladin ; rage drain - great combo with chargers ;) , should be high lvl.

1. Zerock- Underground Blades
2. Sleem- Poison Spit is worthless, but use it as cheap way to level up Sleem. Boost Glot's Armor for later as all the other abilities are pretty much worthless late game and the armor can at least protect your tank, preferably a troop of Emerald Dragons
3. Lina-Just get to Gizmo and level it up. the rest? meh
4. Reaper-rage Draining. The rest is meh