I used a save game for that.
I was bummed to find out how certain unlockabl3 were dependent on playing through multiplayer which is impossible to play currently due to IO Interactive shutting down its servers.
So I downloaded a save game specifically to get them after I played through single player to unlock everything I could that way then put in the save to unlock the multiplayer only content.
When I loaded up the save, the achievement automatically unlocked because, in addition to all the multiplayer only unlocks, the save creator had also maxed out all the upgrades for the weapons too.
So that's how I got it.