the teethgrinder. lock on, fire,  B X Y!!!! 

I was able to get the dentist to lock onto the pistons, then I just sprayed and watched them go down. Just keep them in your view as long as possible (even when dodging) and you should be able to lock on.

Honestly, this is the most bullshit part in the whole game, one of the only times I died on Normal. I ran away and locked on with the The Dentist until I was out of ammo and then relied to using the Hot Boner's alt fire bomb thing. 
 
Fighting those guys is my personal vote for worst QTE in a game ever.

Just alt fire on them with the boner, rinse and repeat. Even on Legion it only took about two hits. Really not that bad when you ge the hang of it. Don't waste time with the dentist the pistons soak up way too much damage.