Serious Sam: Siberian Mayhem

Not a guide per se, but just a heads-up. You can use unlimited ammo for this achievement, which makes the game substantially easier. Start a new game, select the first level, and in the difficulty level selection press . Once there, select Serious difficulty and check the "unlimited ammo" box.

Follow the secrets guide in order to get several of the broken pickups like Serious Time (slows down time so you can get kills/rush through the level easier), L.I.F.E.s (heals 50 HP), T.A.C.T. (artillery strike), the Hoverbike (very fast vehicle with dual miniguns) and the mininuke (kills enemies on sight) and rush through the levels spamming those.


For the final boss fight, just spam 5-6 T.A.C.T.s while dodging his attacks and use the hover bike, which makes you invincible. Do this 2-3 times as needed.

I completed the game this way in one sitting in about one hour.