Crude experiment here. I ran both Mistuba and Tiki solo against Expert regular brunhilda. All runs used a MUB Fatalis dagger as a control weapon.

The first run was a basic weapon and Leviathan while the second run was used with the optimized setup for Mitsuba and a "optimized" Tiki (chocolatiers).

In the former setup Mitsuba cleared in 1:01 (not optimal since I let AI control both). Tiki did horribly until transforming and cleared in 1:03.

Adding chocolatiers made a massive difference; Mitsuba ran Twinfold bonds, his clever brother, and Siren, clearing in 1:01 (again, AI so not optimal). I expect player controlled will clear faster. I did manually transform into siren to boost dps.

Tiki utilizes Twinfold bonds, Choclatiers, and Leviathan. AI immediately transformed and used both dragon skills (always fully charged on transform) and nuked Brunhilda clearing in 21 seconds

Tl;dr - This test isn't perfect and neither run was optimal. Tiki as a dragon appears to do significantly higher damage than I had expected and does look like (outside of team buffs) is outclassed in damage and tankiness.