DO as I told you.
Abe can:
- walk - his normal move
- run - hold shift in the PC version
- roll -crouch then use the arrows
- and SNEAK - Abe walks quietely making no noise(this way sligs can't hear him) - HOLD the ALT key (PC VERSION)while using the arrows
So, sneak a little to get closer the slig (he won't listen to Abe), crouch and then throw the rock.