Double Tap is the big buff you need to watch for.
Every true shot, which is itself kinda scary but the double tap it gives you is the real killer, and volley gives us a Double Tap. It makes our next aimed shot or rapid fire do an additional shot/shots for 80% damage.
That along with a kill shot proc or buffed arcane shot is the big scary burst, two aimed shots and a kill shot landing at the same moment is what causes the one shots.
Outside of that literally just stop as many aimed shots as possible. Everything about MM is tied to aimed shot, it gives a buff that makes our instants hit harder, it resets cds on rapid fire, if you stop a MM from free casting aimed shots he’s gunna have a bad time. LoS, CC, run up on his face and run thru him behind him right at the end of the cast, etc. anything you can do to not let them aimed shots fly.