Infinite Rage and using Lina&Reaper

[ender]

experience of spirits is dependent of leadership of enemy troops left, level of hero versus level of spirit and other things.

Is best to use spirits when enemy army is strong to gain more experience.
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.