The mobs are projected on a very simple quad (two triangles should do it) and you can make them always face the camera using a lookat feature (rotate). The sprite animation is a feature of ue4, you can learn it over youtube, and that’s it. The reason you haven’t found much info, is pry because you had a wrong assumption. Both doom and duke was never 2.5d, but pure 3d games (it actually is in their official name as well, doom 3D, dn 3D) with sprite enemies :]