A sniper what? A sniper round? You need to define it more than a sniper what caliber.
50, the only one I know of.
Hoping to snipe down 4 inches with one seems out of the question without weakening severely, so she wouldn't really have a hope to win attacking directly, doing more environmental damage. Environmental includes him getting stuck somewhere, somehow, and getting shot multiple times in the same spot until the sniped diamond gets to the weak point. That is unless he can repair the damage on his armor, (which it seems like he can? I can't really find much info on the character), in which case she'd have to abandon that plan and go for the plummeting to his doom plan or hoping she could outlast him up close and shovel away small chunks with each swing.
Therefore, she doesn't really have a strongest attack against him, just as long as she can do a bunch of minor attacks for chipping damage.