I have made a video for anybody stuck on the part where you need to kill the 2 snipers. Also if you are having trouble with the mission here are some cheats. Cheat codes do not disable achievements, but you need to enter them when you die, restart checkpoint, restart game, or continue to next mission.
Invincible cheat - HOLD RT+LT+BACK then enter
Y Y X (RB) X (LB)
Invincible Team - HOLD RT+LT+BACK then enter
X X Y (RB) Y (LB)
Unlimited Ammo - HOLD RT+LT+BACK then enter
(RB) (RB) (LB) X (LB) Y
Life at 100% - HOLD RT+LT+BACK then enter
(LB) (LB) (RB) X (RB) Y
You must be holding RT+LT+BACK while entering the cod. Make sure to take your fingers off of RT+LT+BACK once your cheat is typed in. Repeat for each cheat. If you hear a beeping noise it means the cheat has been successfully activated.
If you have any questions on how to complete the mission or about the mission feel free to ask and I will answer. :) Good luck