Board Thread:Questions and Answers/@comment-27327020-20160404174206/@comment-373131-20160504233509

In that line, technically, you DON'T beat him. You are physically incapable of hurting him, or even moving. So Alphys's line is entirely correct that Frisk cannot beat Asriel. The only option is to survive him until he doesn't have the heart to keep attacking.

Which is what Frisk does.