You'll need the 2nd DLC installed to do this and not have finished it. I was struggling with the the various boosting methods for Chaos XP post game. Sure they worked but the XP gain was excruciatingly slow. At the Infestation at the Lake mission in the DLC there is a spot where you can get 2000-10000 XP every 5-10 seconds which should shave countless hours from the nonsense level 14 trophy. To start make sure you have a PBX Mine Launcher and a AT3 RPG equipped. Fast travel into Lago Baja Qacha ( Lake Infestation mission is on top of it) and head to the SE corner right outside of the big orb that contains the monsters. You'll see a bunch of Black Hand soldiers fighting monsters on the inside from just outside the orb and 2 enemy helicopters. Grab the attack helicopter and blow up the other helicopter and the enemy tanks (sometimes 1 but usually 2). Fly the attack helicopter into the orb which instantly causes it to crash and then just grapple parachute in circles above the enemies going in and outside the orb. Every 30 or so seconds an enemy helicopter appears you can grapple hi jack and crash into the orb. As you are continuously circling this area infinite enemy vehicles will crash into the orb giving you thousands of XP every few seconds. If you die you respawn in the same area so you can repeat until you hit level 14. Use your explosive weapons on tanks that occasionally spawn as well. Good luck!