The spirit skills give your monster a significant boost in a particular stat. This boost is not visible in game, but with the below calculator, you can determine exactly what your monster's upgraded stat really is. The stat is effected by your monster's cost, intelligence, and skill level.
***Needs data to be more precise for costs other than 2.0, but shouldn't be off by too much (2.5% at most based on aura formula).
*Special thanks to Arc Tech for developing this calculator!