Semper Fi or Die is the 10th mission of Crysis 2. For this walkthrough, players will be joined by a group of US marines and battle through dropped alien attackers, including both normal aliens, Tanks, and an Alien gunship. The last part of the mission is significant for acting as a gauntlet, with waves of enemies of increasing difficulty.

1. After the cutscenes, the player will wake up, surrounded by a group of friendly marines.
2. Take the gun from the marine by pressing F as suggested.
3. You will have to help the marines make it to the extraction point. Make sure to stay with them isntead of running off forward.
4. Pick up the ammo and guns.
5. Continue following the marines.
6. The aliens will only start appearing in front of you, so don't worry about your back or the sides.
7. Start assisting the marines with the aliens as you move deeper in.
8. Pick up the ammo again, just under the large group of aliens above.
9. Continue picking up ammo as you move with the group. You should be depleting bullets quite fast as aliens take a lot of hits to kill.
10. When the alien gunships appear, you will need to start moving more quickly. Follow the marines, and start watching your back and sides, as aliens will start dropping down from all around.
11. Note: when you finally reach the objective building, you can use the C4 from the nearby ammo cache to easily take out the group of aliens ahead. Just run up, throw it in, and detonate.
12. Be sure to pick up the JAW rocket launcher. You will need it to take out the gunship ahead.
13. Move on ahead after the mission update. The marines will hold the position while you clear out the area ahead.
14. The alien Gunship will appear. Shoot it once with the rocket launcher, and it should run off.
15. The next area is crawling with aliens. You pretty much have no choice but to clear them out. Make sure to make use of Armor mode, and melee attacks when an alien get close to you.
16. Continue on. Another large batch of aliens will drop down. Clear them out in similar fashion, trying to isolate them 1 by 1. Fall back if you get overrun to reload, recharge energy, and get your health back.
17. Pay special attention to the Tank alien. It's a good idea to leave him to last, and to use the environment to bait him into a good position. One effective strategy is to lure him back to where the 2 yellow containers are, and duck up onto the road to shoot him, and then duck down to avoid fire and reload.
18. Continue on to the objective.
19. Make sure to continue resupplying whenever you see new ammo caches.
20. Jump down onto the roadway below you when the mission updates. You will have to go all the way around to get to the objective, as there is a huge drop to your right.
21. Get in the car and start driving around the large pit in the middle.
22. Run over any aliens you see. They will die quite easily.
23. Make sure to drive over any large road cracks in the safest spot possible, or you may get stuck, or worse, fall down.
24. When you see the road ending, don't be afraid to jump across in the car.
25. Get out of the car after the jump. Take the ammo, and continue on foot.
26. When you get to the evac point with the marines, a large number of aliens will drop down. Take cover behind the various wreckage with the rest of the marines, and take down the aliens.
27. Make sure to continue resupplying on ammo and weapons. There are a LOT of ammo caches around the area where you and the marines are taking cover.
28. When the Gunship appears, instead of trying to fight it, simply run through towards the objective point.
29. There are even more ammo caches here. Just save the Swarmer gun for the Gunship later.
30. Take out the next group of aliens.
31. After a few waves of aliens, the Gunship will return. Pick up the Swarmer gun, and use it, in addition to the JAW rocket launcher, to take down the Gunship. 
32. Move up towards the marines, and follow them.