Shield bash is a move unique to Robin. Press 5 twice quickly to use in combat. It has no effect on shielded thugs. I found it hard to aim, but mashing 5 works.
If you don't have it yet, during the predator battle with Harley, thugs will keep spawning to ensure she has two at all times. Lure one into a corner, drop behind him, shield bash, then ground takedown him and flee. Repeat five times to get the achievement.