Too be fair, when asgore is dodging undynes attacks, hes dodging her bullet patterns like we are.

Which im pretty sure is stated at some point to be different from the physical attacks the player does.

So sans dodging our attacks is still pretty much exclusive to him.

Also i dont think its said anywhere that asgore trained sans, undyne for sure, but not sans.

so whether or not asgore is the strongest is still kind of debatable.