Look here, when Kerkovian died:
Only lightning pulse was an action used on ???. In the same WU he died.
In page 331, Flower complains someone sent a kill her way, a minute later Fuji posts this write-up:
Lightning pulse was likely used to kill kerk, yet it failed against flowa. The other possible killshot is tentacle smash, but nothing implies that action failed.
Anyhow, lightning pulse is the likely killshot here.