Now, Severus is inclined to fight back. There's a scramble - putting it lightly - his head rings and he tastes blood, and suddenly there's a disorienting shifting at his back and he realizes the lift doors are opening. Severus shoves Sirius again, hard, pushing him towards one side in an attempt to leverage the sudden space behind him, twisting to try and throw his balance off and get him to fall and let go. It's unlikely to work, but it's something.

Regardless of what happens next, whether Sirius falls away or whether he keeps clinging on, throws another punch, whatever, Severus follows it with a twitch of magic that's not articulate enough to be a proper spell but strong enough to force them apart no matter what Sirius does.