Tips : Brunilda's main tactic is spawning alot of carrot enemies and trying to overwhelm you with mass. As long as you're faster than the carrots this is a really easy boss.
If you can't get away from the carrots try to bunch them up and use an Area of Effect weapon (Porcupine Bombs, Dragon's Bile for example)
If you don't have AoE weapons and are too slow to get away from the carrots your best bet is to try and nuke Brunilda down as fast as possible.