fast serp. But yea, in order to pull this off consistently, always make sure that you are just left of (or slightly on) the rock in the foreground and do all of the standing kicks there. Once you stagger him, you get in one more kick then immediately go left very briefly then boost right. If the position, movement, and boost are done correctly, the windup will hit serpentine, then by the time the boost comes out, it will hit serpentine in the exact position for him to face right instead of left, causing him to go straight back to the wall. You can also get an extra kick in after the 2nd hit of the boost. Then just rinse and repeat.

Edit: I actually just noticed that I was too close for the 2nd boost but still killed because the first boost was able to get 3 hits in, need to do more testing to see how consistent getting 3 hits is with this setup.