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Secret Achievements Human Plug: Go to Caverns and stand over the Geyser for a little while. Embarrassing: At Foundry use your killstreak and instantly die in the steam of lava in the middle of the map, you must die from the pouring stream of lava and not the pool of it below. Waterwings: Drown in the water at Village by going to the sign and crouching in the water, you should start rapidly lose health after a while. Detective: Go to level 5 on campaign and go to the big black wall on the right, crouch and go through the wall via secret passage and go to the body.
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BEST way to get XP:
Go to the Challenge level "Golf Season"
Go to the very back of the plane
Shoot like a maniac! (They can't hit you there)