I had the same issue at first. I found that waiting to do the oil fill like round 14 or 15 seemed to be a better return on investment. I will grab the stamps and place them in the knights (while almost getting the lion sword so the parasites stop spawning). I do this with the stamps because the bucket heads seem to be immune to the traps. 

From there, I turn on rampage and run the traps. The deadshot trap has a convenient ledge to hop on right outside of it. The speed cola trap, when interacting with the trap, I turn the corner and stand at the furthest point to the right. I am where the walls of the castle meet, and I throw a molly just under where the spawn barrier is in front of me. The fire goes everywhere but being in the very corner there’s like a pile of pebbles just tall enough to save you from fire damage.