You just need to adapt. I always run engineer with a black hat or EMP grenade when playing FFA. I use Betty's with scavenger and jut leave them in random places I think people will pass through. Maybe it's a bit of a cheap tactic but I'm playing to win. Sounds like you and the other players were feeding that guy kills, you should have just avoided him. Easy enough to do if you know exactly where he is, I've not seen someone win a game by hugging corners yet. I hardly see any UAVs in FFA, seems harder for most players to string enough kills together.
I completely agree with you, this game overall is just a huge disappointment. The spawns are by far the worst I have ever seen. If you kill a guy more than likely he ends up spawning right behind you and it's no surprise he's just going to come back and shoot you. And when you get killed it spawns you right in between three people because apparently that's a "good spawn"
Very disappointed with the multiplayer it is completely camper friendly and the delay in this game is mind blowing. I will be completely around the corner or on the other side of a wall and still manage to get shot.