Stick with growl

You doubt Charmander could even scratch him in this position.

"Charmander! Use growl again, but louder!"

Charmander lets out a more powerful growl. Still not that impressive, but you can tell it affected Squirtle a bit.

"Again Red? Don't you ever learn? Squirtle! Tackle!"

Squirtle stands up off Charmander to tackle him again. This might be an oppurtunity for Charmander to get a quick scratch in, or you could have it try to dodge to get to safety.

Squirtle's HP 100% (attack lowered 2)
Charmander's HP 80%