On the list of corrective exercises at ExRx, the only one that specifically targets the serratus is the Incline Shoulder Raise (shrug). The other exercises might be targeted at strengthening supraspinitus and external rotation, which they list as weaknesses that make the problem worse, but IMO there are better choices there (whether you want to go with compound or single joint exercises). Another exercise that I have been fooling with that might
hit the serratus better than that incline shrug is the overhead shrug. The scapula is not pressed against a bench. See
This is an unusual movement, everyone seems to have a tough time keeping their elbows locked. I have been trying it one-handed too.
Overhead presses also strengthen the serratus but IMO direct work first is the way to go, although reasonable people disagree on this (that's another thread).