Ok this is the strategy I used to complete the level 50 achievement (Serious Complex) in a couple of hours I started at level 24.
Use the Gold Coil Rifle and make sure you are playing on Insane.. FYI any Unlocked Gold Weapons will give you double XP, so use whatever you are most comfortable with however I prefer the Coil Rifle as it's best for headshots (which will still give you a multiplier). Next you'll need the Grapple Hook, the 2 Thrust Packs, and Missiles you can however alternatively use Unlimited Missiles which makes this strategy a little easier to do.
Near the top of the map you'll come across another lake (not the main one with the cottage), near the gold guns secret room.
Save your game in there, then exit to the surface and run up the side of the mountain using your grapple hook. There will be 3 enemies who you can melee or if need be headshot for a total of 5x experience bonus. Next run to the left and use your Inertial Dampner (not required) and double jump down the side of the mountain. Once you have hit the other side of the dock there will be 3 more enemies. Melee them and you will come to a WSA3 Devastator, one of the walking robots. Leave him for now and jump over to the left and another gatling gun enemy will be present, once you kill him your combo should now be at 9x. Now go back and finish off the WSA3 Devastator, with a missile strike at it's back or undercarriage, for a Max Combo. Killing this nets you a high amount of experience. Now, run back the the save room and once the game is saved, simply hit start and select 'load from last save' to have the Devastator respawn. Rinse and repeat.
I did not create the video and this is somewhat the same strategy as the one I just posted. Use this video as a general guideline.