Completed in 7 hours. Some tips:
1) Focus on getting XP. Forget about money and everything else. If you can get 2 experience rings, always wear them.
2) Upgrade bunnies to max. They are the most useful unit in the game, because they attack at distance and can knock back enemies when in aura.
3) When leveling up, I always picked up abilities to attack: attack rate, more damage in aura, special attack more often in aura, restore health of units in aura, production time, mana cost and the most important, food and mana production.
Forget about Paladog abilities, cover behind the bunnies and you will not have any problem.
4) Stages based on luck will freak you out... specially 4-03 and 4-15. You will hate the game for that.
5) You can level up 15 levels in the stage 5-19. Just make bunnies and wait for giant mummys. Keep killing small mummys to get infinite XP. I leveled up from 102 to 120 in my first play.
There is a HUGE bug in the game. To do it, you must have Charge ready (Horseman gallop). When Horseman is about to die, do the charge. He will die. When resurrected, he will continue the charging... and will never come back!
I dont think the final boss its too difficult. The key is to always have defense or counterattack. And when you got a rainbow use it on a Special Attack. It deals 200+ of damage. I completed it on the second try with this tips.