It seems like you have the code calculating XP written wrong. % of killed enemies currently is based on the total number fought (not total per level). Therefore I can play like three rounds, let them take my gems, and then collect xp of 66% (20/30) + 25% skill bonus. Basically get the full level xp in 5 min allowing you to max out your skills very easily and early.
Game is pretty easy when you realize you can equip any item in any slot and still get the stat bonuses (read bug).... Also, after lvl 10 the game gets boring as hell.
Try to beat the level Are You Pro?!!! by parrotparet... HARD AS HELLO. Currently ranked 7.5 because no one else can beat it but me. HAHAHA
(on page 3 or 4 of highest ranks)