It's all about placement, mats are limited so if zone moves too far get down and relocate. Try to get max height before Doom sees you so that he cannot shoot you down. You need an AR with sniper or thermal scopes (I prefer sniper scope), max mats, meds, med kits are best and a get down. IMPORTANT...don't engage until its 1v1 otherwise remaining players with guns will probably shoot you down.