I used this strategy to finish this trial:
Go to the open area on the left side (near the gongs) and wait for zombies to crowd around you.
Charge Flame Dragontail (L2 + X) for a few seconds so that your enemies fly up into the air.
Let go of L2 once you get into the air. Next, hold the Triangle button..do not spam it. This should get the job done of the first try....goodluck!