Attempting a somewhat fast song. I can get it down sometimes but sometimes I can't. Would it be better to practice exercises to target all my fingers in hopes of tackling this hard song or just keep practicing this one particular piece?
in the long run, just practicing should be exercise enough.

If Its a "somewhat fast song" and your having trouble playing it up to speed, try working it slower.

Just turn the tempo down a little bit and play it as steady as you can, and get comfortable. Practice at a comfortable speed and turn up the pace little by little every day.

This is how you master a "peice"