Anyone remember hero-kun's shell amount? It's possible his skill is contingent on the number, like he uses a percentage of the overall shell, it drains a thousand points, and does less damage without Rude there.
For everyone hoping for his death, his skill is a perfect fit for mc's skills too, it wasn't just one way. His Life Burst drains Rude's shell, to do massive damage, and then Rude turns that damage into another attack of similar damage.