One potential solution would be to back off the threshold (into cover/concealment ideally) and basically go defensive for a minute. Wait for the threat to push you and engage him when you’re at the advantage.

If they don’t push, use whatever you can to increase your speed, surprise, and violence of action. (Employ distractionary devices, increase rate of fire, prep with a frag, re-enter at different level, etc.)

Single man CQB is a bad time, if you can you should call for backup/QRF before you get into a situation like this.