Rage Spirits
 

Is it really worth developing the rage spirits? you can't use them anyway when you need them IE. boss fights.

Well it doesn't hurt, does it? Why not use something is just going to go to waste otherwise?

@scarletgamester Ofc it is worth it man, you have 4 spirits with 4 skills each. Some are dmg skills some are supporting but every 1 have it`s use. Usually warrior use them more because have lots of rage at disposal (i almost every game make 30-35% overall dmg with them, not counting usage of supporting rage skills). If you are new to this game, give them a go and see how it turns out, but trust me they are worth using.
Have fun :-)

Considering there are like... what? 3 bosses? 4? And all of those except one is optional?

Yeah, they are worth using. Reaper, especially. I find Zerock and Sleem underwhelming, personally, but I do like Sleem's support skill. Lina I'm on the fence about.

Lina is arguably the best, even taking into account Reaper's "Time Back" (which really is awesome). The orb gives you a level5 free target, inasmuch as nearly every critter will choose to attack it rather than your normal army.

Much more significantly, she has the Chargers.
The ones for rage just mean that they are mitigating the rage cost of using the ability, while the ones for mana help you convert rage into mana.
But perhaps most importantly, they let your armies MOVE FASTER, particularly when you upgrade her to the point where she's putting out 6 of the darn things.
If you happen to have level3 Haste then some of your troops who walk across the Chargers can wind up with a net +3 (or more!) to their effective speed, blitzing across the map to crush your enemies.
The *only* down-side to the ability is that they come out in random placement.