You can always grind a couple of levels the day before the boss fight. 9th dungeon floor is the one that should give the most experience. Try to get old porn cards at the minigame after battles to get more experience, but apart from that the grinding is as it is, sorry xD

Here's a video of the boss fight at level 11 (it should be quite easier level 12): https://files.catbox.moe/4ddfme.webm

Colorao's 2nd battle strategy:

• Colorao attacks with lightning type attacks, so the MC should try to equip a Cirote resistant to this type of attack.
• Colorao is initially weak to fire, ice and lightning attacks, so you can attack with the MC and the Yona while the Mamen heals. If the Mamen can't heal enough, let the MC help her.
• When Tomatico (Colorao's son) appears, Colorao is invincible, so you have to kill his son as soon as possible. He's weak to ice and fart attacks, so the MC and the Mamen can attack him. The Yona can also attack when Tomatico is stunned on the ground. The goal is to keep the son on the ground as much as possible so that he doesn't help his father.
• Once his son is dead, Colorao becomes resistant to fire type attacks, so the MC is the only one who can knock him to the ground with ice and lightning type attacks. Let the Mamen heal and the Yona give support with Tomato, heal with objects and attack with ice and lightning type pills.