Should I burn my fifth copy of lancelot(zerk)?

Np 5 him, 1 prism is nothing

Depends on your needs. If the rare prism gets you a Bond CE from shop or something those cost 3 to 5 rare prisms by all means. Otherwise NP5 him or hold onto the copy if you wanna wait for something better to get stocked.

I burned my 2 stheno dupes and fionn to get a heroic portrait CE for bomd farming personally.