If I were you I would definitely put my money into training before fighting. Although the Koi Warrior only has 65 hp, your pet probably only has like 36 hp max. Koi Warrior can attack with a 12 icon weapon which will cut you down rather quickly. You may just want to stick to fighting opponents like Chia Clown and rack up a bunch of codestones for training.
Your best choices for damage are gonna be any of the cheap Brightvale scrolls: Scroll of - the fool, ultranova, knowledge (most expensive). Sun and Moon Chakram is good too. Two Scarab Rings would probably suffice too.