This is not so complicated as it sound, since you don't actually play a complete level as the description might apply. The condition is meet even if you start halfway through a level and then reach the boss without getting hit.
The best level to do this is Level 3, Leos Place. Just start from the "Stage Area" checkpoint and win the fight without getting hit. To win this fight quickly, your character should be at least level 12. It can be even easier, if you use the cheat code for the sword of love (press
on the title screen).
Once you breached through the backstage doors and get thrown outside the trophy will pop.