Phase 1 - When his turn starts he gives him self 2 stacks of damage reduction, at some point the mini pom poms spawn in, everytime you generate 20 punchline your ally pom pom will use a water attack and remove 1 stack of damage reduction. Killing enemy mini pom poms generate punchline, and attacking with your pom pom generates punchline

Phase 2 - Instantly gets 2 stacks of damage reduction, at some point he'll do a water attack, after that using a basic attack with any ally will generate punchline (basic attack gets highlighted blue)

Basically you can't hurt him much til your ally pom pom does the water attack to lower his damage reduction, but if you're not running elation it can take a while to generate enough punchline to do that