The trenches can get really crazy really fast. There are basically 7 main waves. Each wave hits a small area of the bunker. These areas are separated by the machine guns. Look out to the horizon and watch for the waves. The first 2 might already hit the trenches by the time you get into position. If so, then just finish them off and start looking for the next one. They steadily move to the right, so just watch for them. If you have time, then jump onto the machine gun by the position and gun them down. If you can't do this, then clean up whoever makes it to the trenches with your shotgun. It is automatic, so feel free to just let loose with it. You won't really need to save your ammo, since this is basically it. Cut back to the left when the trench offers a path forward. There should be 2 more big rushes. Once you take out enough of these guys, they'll fall back to regroup.

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Go out of the trenches and up to the hilltop to meet with Dexter. He'll give you another new toy to play with for this wrap-up. You've got a laser-guided grenade launcher. You have more shells than you can physically use, so keep firing. The shells go into the air and then come back to hit the spot where you laser is pointing. The trick is to keep firing while you also move your laser across the German army. You don't necessarily have to kill a certain number. Just keep laying shells along the line up to the bunker. You'll eventually scare them into retreating.
Go back to Dexter and report in. You need to join up with Petrovich and get him out of here. That would be too easy though. As you approach the marked spot, some agents will spawn and things will go downhill fast. Just sprint to the spot in the clearing and get into the portal.