The Brain in Crimson is pretty easy so long as you get into the large antechamber, rather than in one of the tunnels.
In there you can swat away all the mini-enemies, and once they're done the brain itself is really simple.
Especially if you have a trident-type weapon, as they have great knockback.
The worm is a little trickier at lower levels, I find, unless you're also in an open space with ample room to move. A strong trident-type weapon is great too however, as you can hit multiple segments in one go.