Derek first form, you will have to shoot him, then you will have this scene where you have to run from him on the other train, shoot him a couple of times and then you will have to shoot him from a distance before he rams the train. Derek second form, this will be a long fight in his T-Rex form. Run around and shoot explosion barrels and hit him. Ada will support you in this fight. Derek third form, this one might be a pain in the ass because you will have to climb a rope before actually fighting him. Just shoot him and hit him. Finally his final form, seems pretty easy once you fight him fully, just use the electricity rod, it should be laying around and impale it in a zombie so that Derek will take it and hurt himself with it.