Unfortunately Plesiomon's ability doesnt seem to do much in the way of damage despite being a def peircing attack thanks to it being AOE. I've never managed to out damage the healing they give themselves on their next turn and I can never seem to damage 1 of them enough to kill them in a single turn. Its absolutely maddening. But I am playing on hard. So if you're on normal you might have better luck.
Maybe try adding acceleration boost on dps mons? I pretty much relied on Lilthmon for the whole game until imperialmon PM and master cup. I was playing on hard too.