Knocking out the maps on master.
Progress towards the achievement? Nope, you need to be in the lobby before the game starts.
Ok, that`s what I thought. Thanks.
Can you still start before 50 though. ?
I mean new game but starting at a higher wave ?
According to this it looks like you can.
It worked that way for Seriously Chapter 1 where loading wave 50 and finishing it counted towards the achievement.