First make sure you or your partner has 2 M3s and 1 S&W magnum. If you were unlucky enough to get 2 M3s then a SMG like the Scorpion or the MP5.
What you want to do is first is to smash open all the containers near the area and grab all the ammo and health items, but don't trigger the cutscene while doing this. After you have collected all the ammo and health items then trigger the cutscene.
When the timed fight starts head into the little room with the hole on the top and grab the ammo inside. You and your partner must then hunker down in a corner after all the ammo has been collected, but make sure that the corner is not near or below the hole. You want to be in the corner near the door that is away from hole and make sure you on the right corner so that way it can open and you can see the enemies, standing in the left corner will make it worst as the door will open on you and you won't get a good shot at the enemies. 1 person must shot the enemies that are coming through the door and other other must shoot the enemies coming in through the hole and the door on the other side of the room.
When the bosses arrive then both of you must focus fire on the boss rather than the regular enemies. If you get the Chainsaw Majini then they should be a piece of cake if both of you focus fire on them. If you get reapers then both of you must keep firing at them. If you get the Executioner Majini or the red one (I can't remeber if both varients will spawn during the fight) then focus fire on it, but if 1 person dosen't have the S&W magnum you want to move around the room so he can't get you, but if you are lucky and Executioner decides to take the stupid route and jump through the hole then you shouldn't have any problem, but if he comes through the door near the corner you 2 are guarding then move around the move so both of you won't get pinched.
Just stand in that corner and defend yourselves from the enemies and bosses until time runs out and you will be done.
Also when you and your partner has time, try to grab ammo that were left behind by the enemies, but make sure that the ammo is inside the room.
I know that this may be kinda of hard to understand. If you need a better explanination of this I will try to make a little tutorial video.