which are the best answers to get the spirits of rage to work for you

[Metathron]

For sleem, you need to give him something venomous, so it doesn't have to be royal snakes; in fact, if you can feed him the other ones (swamp snakes, I think), you'd be better off doing that rather than wasting the great royal snakes.

Reaper will serve you after you liberate a castle in Taron Mines.