So, we have to do all missions excluding What-ifs either on Hard or Online Versus.

I don't really want to do the missions solo on hard, as the enemy bots are too good compared to your side with allies doing absolutely nothing to win the match.

Thus I'd like to get them via Online Versus, but the option is greyed out on PC. How can I obtain the achievement then? I tried Campain coop, it didn't work. Will Freeplay Versus work ?
The best way to put it is this then : what type of server should I be playing on in order to get that achievement eventually? (Objective Standard, Objective Custom, ...?)

This is a really good question, and my topic asking why online versus is greyed out fell off the front page without any replies. Notably, online co-op can be played on hard mode, but I'd really rather play against people.