After the cut scene Marcus is back in the tomb. Go straight and then turn left. Head down the hallway. When the hallway meets a big room, move over to the left side of the room.
Head father in to the room towards the door. Go through the door. The Berserker will burst through the wall. When it charges you wait until the berserker is about to hit you and dodge out of the way.
After the berserker misses you run through the door on the right. Run all the way to the end where a door is. Stand in front of the door. You may need to make a sound to get the berserker to charge you. Wait until the last second and dodge out of her way. She'll smash the door open for you.
Get around her and go through the newly opened door. There will be another door to open in this room. Run through this room to the other door.
Get the berserker to charge Marcus once again. Dodge at the last second to have the berserker open the door for you.
Once open get around her and through the door. Run to the other end of this room. Marcus will find another door and a timer will have started.
Get the berserker to charge Marcus again. Dodging at the last moment so the berserker will open yet another door.
Get around the berserker and go through this door. Marcus will be outside.
Get the berserker to charge outside and dodge her once again. Once she's outside aim the Hammer of Dawn at her. Shoot her with the Hammer of Dawn. This will leave Marcus at the end of Act 1.