Crouch and go up behind the guard until the Y prompt comes on screen. Beware though, once you take one guard down the rest flock to the unconscious one immediately.
In the medical centre I take to the gargoyles and batarang from afar while constantly changing gargoyles. If I can, I set up explosive gel and glide kick+ground takedown then flee to a gargoyle again. --- Winners never quit. Quitters never win.