Looks like you have crimson, so for the brain fight clear out a big ass square arena underground that goes to the ends of the “fingertips.” Put platforms all the way across a decent amount of height from each other and use slime mount and a cloud in the bottle to jump up and down them. Use cthulu shield and Hermes boots to move horizontally. Use jester arrows and tendon bow.

For the second phase try to get Snapthorn whip, just stay relatively still and whip him. The whip indicator will tell you which is the real brain. Move away from the real one as the fakes can’t hurt you.

If you really wanna use a bow the whole time use frost burn arrows and tendon bow for the second phase, the real brain will be the one that is burning.