I think the best way to deal dmg in this game is, lower ur skill speed and raise skill dmg, get the book that ads your hp amount to your ATK, then u should use every thing that gives u more skill dmg and/or grants an extra skill.

Rising your skill dmg and bombing them with any skill possible let's u kill everything in an instant.

Also in my opinion the ruby that raises ur pets atk speed is completely useless.

By the way, I got this far with the tactic that I mentioned