This achievement is cumulative over all your Version B plays with all characters -- once you've earned 5 platinum medals total, this will pop. Fight near the top of the screen when you can, do your best not to die. Conserve bombs for the bonus score at the end (but go ahead and enable Auto Bomb if you have trouble staying alive; it's better to drop a bomb rather than die and lose a bunch of score multiplier as well as potentially losing weapon powerups).
You can do this in Bancho mode if you don't want to attempt playing through the entire game repeatedly for this; if you're having trouble getting platinum in stage 1 or 5 or wherever, you can simply redo a different stage over and over.
For me, stage 2 seems to be the easiest to get a platinum medal in.