The best XP farming method goes as follows:
> Create private match
> Gamemode: Rescue
> Map: Beach
> Faction: Survivors
> Set time to 15 minutes
> Set med packs to win to 20
Now all you do during this round is to pick up med packs and a few salvage crates along the way. Rape that sprint button (
). You should have all 20 after roughly 10 minutes. That'll net you between 26.000 and 27.000 XP depending on if you drop a med kit every now and then. My standard amount was 26.650 XP and about 3.500 salvage.
After the round is done and you're in the stats screen you'll quit to menu (without any penalty for quitting) and start over.
Shouldn't take longer than 2 rounds to Lvl. 10. The Trophy will not unlock during the match. You'll get it when the round ends.
I know there are faster methods but with this one you can work on the Trophy whenever you want and don't have to wait for other players.