My advice would be to use mysteries to kill the Dog of Hell as soon as possible, and then stop using mysteries entirely and let standard attacks kill the Chameleon.

The Chameleon has a very high chance to dodge them and I'm pretty sure one of the mysteries is spammable but consumes the Toads standard attack, which is how people often get stuck here.

Can't make the Toad any stronger, no. It's a set battle. Just target the Dog of Hell and use the 'Prompt' mystery the Toad has (never use the non-prompt one).