I think all back movements are going to help build your serratus. Pull-overs are the absolute best (at least for me some may disagree) for targeting that muscle group.
Also, depending on how you position your body chin ups (palms facing in) hits it nicely.
load one end of a bar bell, put the other end in a corner. bring the bar up to your chest. let it drop down to one side, the pull it back up to your chest, then press straight out, then back to your chest. Then, let the bar drop down to your other side, then pull it back up to your chest, then press.