You have 5 min to kill the boss, time counts from the first person who enters the room. (maximum 5 ppl)
Don't use SD or any energy attack, it will heal the boss and his summons.
First focus on Cerebellums, they use area energy attacks and heals the Brain Head. They won't respawn again after death.
Don't forget about "Bad Though" summons, they will respawns again and again until Brain Head died. Kill them before they approach the main boss and heal him with energy attacks.
Continue attacking main boss and killing summons.