1. Equip a stat-boosting perfume, or at least uneqip the ero one you have. This is hugely important for bosses since it's a 2x or more ero damage multiplier.Cool. I'm looking for a way to reduce sensitivity to ero attacks. They build up over time but I haven't seen anything to counter that.
2. For most bosses, lots of fairy whisper + the builtin slashing is plenty to keep your HP up and ero down. As long as you're making the quick time events, you shouldn't have a problem (might fill gauge once or twice if ero sensitivity's built up but it shouldn't kill you)
3. If you need more than that (like on a certain boss that will chain you down if it fills once), equip concentrate x2 (or more if you don't need the healing) and just F12 if you get unlucky on dice rolls/boss abilities.