+1 exp?

[ender]

you can found in translated fan manual more details (and formulas)
seems that first rage use in battle have higher coefficient that 2nd.

those formulas are made for balance of hero level and spirits level - more experience for lower level spirits than hero and less experience for high level spirits until hero catch up.

leveling spirits with weak enemy troops is not working well (I spent ~1 hour for 100 experience)
but you can cast fear on a big single level 1-2 troop and use wall or armour, for example. and inquisitors for rage.