I defeated the big brain tank and then the brain part detached and started attacking me. I can only seem to hurt it a little bit and it's really hard to attack. What am I missing?
What are you using to attack the brain? I found that punches seem to work the best. Stay away from the brain when it spins its beam the fastest, then approach when the beam slows. You should be able to get in a number of punches that drop the brain's health pretty quickly.
Good luck.
Use the leviation ball the bounce over the beam to the brain. Once there you can palm bomb down to it and get in a few additional hits.