Mei will run in first towards the monster with Boothill very quickly catching up to her. They'll likely notice very quickly that for some reason Mei is the one being targeted so he will toss his shield to her, yelling at her to take it because her is "fudgin' useless" and essentially let her be the bait for now.

As he'll definitely notice the cyborgness of the monster, he'll work on finding the off switch or something to slow it down, shooting at it all the wall and dodging any projectiles that get lobbed at him, if any.