Cave, it's still bugged. Got as many as I could legit, but I don't want to go back to the game, so I cheated that one and Destructive Criticism because it's a ♥♥♥♥♥♥♥♥ achievement they've had in some form for years and they still haven't decided to get rid of it so like, ♥♥♥♥ them. By all means, Head Rush should have worked for everyone when you got 3 crit kills with the Harvester in a single life.
I don't want to wait for them to fix it if I'm not invested in the game beyond getting the achievements.
Best of luck to you getting the both of them legit, at least when they fix it.

Hello. Please, guide me how did you complete "Head Rush" achievement in Team Fortress 2 Classified?

the incredibles mr. incredible yeah i've got time obama

Yeah I can help.

yes i would like your help

I don't need the help anymore, but would you like help, MoNsTeRR?