These monsters easily outrun you, but do have a major weak point--and it is not their heads. Aim your gun at their telsa-like apparatus. If you score some direct hits they will explode into bloody chunks.
Hint: Defeating Possessed Hegla
As soon as you emerge from the basement with the ammo stash, the Boss battle will begin. Quickly rush to the gate on the other side of the cathedral's courtyard and plant some dynamite. Take out the other zombies around and go back for the hole. Hegla for some reason will not go in the hole after you. Thus, you can rush over and lob grenades potshot style under where she goes down.
Hint: Defeating Proto-Super Soldiers
Beware of the rockets they fires and the venom gun they carry. The best way to kill them is to catch them around corners with planted dynamite, toss grenades, or use rockets. Be sure to get out of the way after taking the shot.
Hint: Defeating Super Soldier (first meeting)
The Tesla weapon will take you out quickly if you do not get out of its line of fire. Jump the railing to the right and get behind the window. He cannot seem to see you at that location. Blast it with whatever rockets you have left, then try luring him behind the window so you can move around the rest of the room to get the armor and med kits, as well as kill the enemies on the catwalk. You will have to take potshots at the Super Soldier until it starts limping for a better chance at hitting him with grenades and machine guns.
When fighting the Super Soldier with the Telsa Gun, while he is in the computer room under Deathshead, run up the stairs to the catwalk. Stop directly next to the door where Deathshead is talking in that room and get the Super Soldier's attention. When he walks under you, fill his head full of bullets. A few bullets should kill him. This is much easier then continually shooting him in the armor.
Hint: Defeating: Heinrich I
He harder to contend with, given those zombie warriors surrounding him firing life-sucking demons at you. Use the Tesla gun to hit several of the monsters at once, including Heinrich, while circling him. However, this will not be enough to kill him. Once it runs out, use the Venom gun to finish him off.
Hint: Forest level: First tower guard
Ignore him and the two guards at the bridge. You cannot kill them without setting off the alarm.
Hint: Forest level: Getting past the concrete tunnel
As soon as you enter the tunnel, you will hear a guard ask if this is the only shipment. Do a full sprint towards the guard and the truck that is leaving. Duck into the doorway on the right side and quietly open the door. If you do not do this at a full sprint, a second guard near the truck will see you as the truck pulls off and set off the alarm.
The guards at the bridge and the tower can be taken out. You have to kill the tower guard first, then spy on the guards on the bridge until both of them are looking away from you. Then, run up and backstab each of them.
Hint: The Last Mission Set: Stopping Heinrich's resurrection
There is no time limit to finishing this set of missions. You cannot stop the ceremony, so take your time.