"Let's give it our all!"
"Right, we agreed to that. I'll show you the power of my spirit."
"Oh, well instead of going all out against it I'm gonna purposefully nerf myself against you rather than showing you my full strength."
I guess you could make a point that he's still using his "full strength" to fight her with a weak element here, but that feels kinda like calling it "full strength" when a master swordsman defeats someone with a stick or something. Not really full power, just showing off.