I only have a handful of tips leftover from when my buddy and I beat it. Took us around 2 hours to complete (cuz we suck).
-Play in Co-op. Another human player is far superior to the AI... most of the time....
-Keep Flashbangs handy. They will instantly kill all mutated Majini forms. Especially helpful in the first area against the Duvalia.
-Incendiary Grenades can sometimes kill Reapers instantly. I believe it must be a direct hit to the exposed chest. It is difficult to do and doesn't always work, but it's worth noting!
-Knife enemies trying to climb ladders. Conserves ammo greatly.
-Collect everyone weapon you can. Each player should be carrying unique weapons so they only use specific ammo types. Personally, I took the Machine Guns, Shotguns and Magnums. My friend took Handguns and Rifles
-Stockpile ammo and Healing items
-Learn the layout of each area
-Camp in more than just the finale. If you find a good camping spot, use it. Funnel the Majini in and kill them to continue unhindered.
Missile Area tips:
-Pray for Town Majini and Executioners. They are far easier to defeat than the Base Majini and Reapers. As an example, Base Majini with RPGs are replaced with Town Majini with Bow Guns. And noTown Majini will have stun rods...
-If you have the Rifle, snipe the Majini manning the Rocket Turrets. Shortly after killing their original operator, a second operator will man them. Kill him, and a third will arrive. Kill him, and it's done. They will never be manned by your enemies again!
-The moment you begin, there's a jump platform behind you, followed by another small area with a safe. Many people seem to miss this.
-There will be a door that requires both you and your partner to kick down shortly after you've use the Rocke Turret to destroy the first barricade. To the left of this door on the other side is a small area where you can jump down. In this area there is an assist jump which will lea to another ladder, at the top of which there will be a First Aid spray and an RPG. These are invaluable, for obvious reasons...
-Rush through the final area after blowing up the 3rd and final Barricade. It's the home stretch!
The singal biggest tip for this area...
-After the cutscene where you are attacked, to your left (As Jill, Right as Josh, I believe) there will be a ladder onto a container. Climb it. There will be some woodn boards (or something like that) that form a bridge to the nearby building. Cross them. Stand in the far left corner, side-by-side with your partner, between the actual corner and the hole in the roof. It may takes some adjusting, but eventually you and your partner will find a sweet spot here which causes MOST of the Majini to hop up and down impudently between the rooftop and a hole in it off to the left (whoever is standing nearest the hole in the building will have a good view of this). Only a handful will climb up to you. They can be easily dealt with using Handguns and Shotguns. In this position, your biggest threats are the projectile wielding foes (Bow Guns if facing Town Majini, AK-47s and RPG if Base). Dispatch them with the Rifle quickly. Use the Magnum (if you have it) To stun Gatling Gun Majini of you were unfortunate enough to get them instead of executioners. Lacking the Magnum OR after the G.G. Majini is stunned, unload into it with Handguns and Machine Guns 'till it's dead. Repeat on the second one that will spawn later on. Finally, beware Chainsaw Majini (if you face them). They have the highest probability of of not going into "Derp Mode" and instead of hopping up and down through the aformentioned hole, will actually hop up to you. Stun with the Magnum if possible and unload Shotgun and Handgun rounds into it. Beware! It will get back up in fairly short order!
-Gather up all the ammo you can without triggering the cutscene. The Cutscene is triggered, by the way, shortly after passing the container on the roof.