This trophy isn't difficult at all, I'll just give a fast tip on how I got it at first.
Play the game at an unusual time, so that you're alone with bots.
The bots aren't that smart and will most likely rack up in the middle. I could easily get 5 kills. and don't press on their towers, so that you'll fight longer in the middle.
The trophies will pop no matter if you face bots or players. You're still officially online.