1. Play online with a friend. This will make campaign infinitely easier than playing with an AI partner who will probably get themselves killed whilst wasting ammo.
2. The infinite launcher will help you tremendously. I suggest running through amateur just for times; I think I got something around 3 hours, so it should be easy. Other weapons I would suggest having infinite ammo for- Lightning Hawk (it's stronger than the S&W 29 and has a faster firing rate than the S&W 500), the rifle of your choosing (I go with the .222 bolt action because it's much stronger than the others and I'm used to the time it takes to eject a casing. Most people don't like the time it takes though, so if you're one of those I would get the PSG1), and maybe a shotgun. The rocket should be used during emergencies and bosses*. Use the rifle for long distance targets, the shotgun on groups, and the lightning hawk on everything else.
*On the big majini in the village you can shoot them twice in the hip region with the lightning hawk and it will kill them. It only takes one shot with the S&W500 if you prefer to use it.
3. Keep both vests in the inventory. They will give you precious time to heal yourself when you would otherwise die from an attack. Fill the rest with first aid sprays.
4. Wesker/Jill battle- immediately go to the door on the right and trip the cut scene. After it ends, dodge Wesker's bullet if necessary and then run up the stairs and down the long hallway with the launcher equipped. Hide behind the first corner on the left and then shoot Wesker as soon as you see him. You may need to shoot the rocket after he catches it; any gun will do for this. It not only earns you the Heart of Africa, but it also cuts down the first fight to about 5 additional seconds after he calls you a cretan (Sp?) lol.
With Jill you could do what the Major said, but when I shoot Jill's device I always end up killing her. If you're like me, than stun her and then move in and try to tear of the device. It may take a while, but it'll take longer if you kill her.
5. On the boat- go to the first mast and climb all the way up. Ahead of you should be 6 or 7 majini that can/will drop down to the crane and wreak havok during gameplay. Shoot all of them (the one immediately next to the crane switch is completely armored except for his head, so make sure you make a head shot) and then continue on. Where the crane would usually drop, you will find a first aid spray and you can both shimmy on through without the crane dropping. You should also later find 2 additional gems near the twin elevating cargo containers.
When you get to the room where the alarms go off (immediately after seeing Excella), you can avoid the alarms by sniping out the majinis in the correct order. Shoot the highest on the platform across from you whilst staying as close to the entrance as possible, then shoot the second. Next, turn to your left and shoot the majini hiding behind the boxes. Aim a bit to the right and shoot the majini standing near the green herb area. Then run to the end of the railway and stop before it takes a right turn. Hide against the wall and snipe out the next majini. Hide next to the wall he was behind and then aim out into the room. Wait for the majini to come up the ladder and shoot him in the head. You should now be able to run out without alarms being triggered. Use the launcher on the minigun majini and be on your way.
For the final form of Excella, just shoot the orange spores with your rocket launcher until she dies.
On the final run to Wesker, it is VERY important that you stay together (as in the entire game). Sniper rifles will come in handy here. After you pull the second set of levers, have your partner watch for invading majini while you take care of the reapers with you're infinite launcher or well placed mines which can be found in a room on the upper level. Launcher the miniguns, take the cards and run to the door.
6. Wesker- shut off all the lights and have your partner distract Wesker while you shoot him with the launcher. (Use the one supplied to you if you have to. You can also load it yourself if you take out a rocket, set it on the ground and grab the launcher again. It will keep your partner from dragging Wesker to you.) Get fairly close to him before shooting the rocket to have enough time to inject him. If it doesn't work the first time, it will work the second time.
7. Uroboros Wesker- If you haven't unlocked the infinite launcher yet, now would be a great time. It will be ridiculously hard for Sheva to climb the cliff on Pro, so I would suggest shooting Wesker repeatedly with the rocket when you would originally push the boulder. It will take several minutes, but you will see an alternate cut scene from where you kill him with the knives and the game will end. DO NOT let Sheva shoot him if you use this method. He will not die up on the ledge and so you have to trust that you or your partner can make it up that cliff, which I can never do lol.