For the "Not A Scratch" achievement, I did this in the Doom 3 mission Delta Labs 2 I think. It is the level that you first encounter the HellKnights emerging from a portal. It has 4 enemies (2 chainsaw zombies and 2 HellKnights). If you had an assload of rockets and the BFG9000 (Reference "Ready For Action" on how to get the weapon) like I did, it shouldnt be a problem. Blast the zombies with rockets from a distance, then fry the HellKnights with two full charges of the BFG and enter the portal.
Primary Boosting Focus: Ghostbusters, Halo 3 and Borderlands 2