I made a mini guide with tips under hard, which is basically applicable here. I will just jot down a few things I noted on extreme here, otherwise use my tips from the hard solution.
Once again my main suggestion is to do this with one other player; I think maybe you can struggle through most of extreme on solo, but 3-3 is basically impossible with just bots.
1-2 boss - You need one thing and only one thing here, buster grenades. Build up a stock pile of 8, hopefully for each person on the areas leading up to this boss. Once you get to the bosses just let them rip. Aim towards a central area on the boss, dont worry about hitting the head, the explosion of the buster radiates outward dealing damage to the boss. 8 grenades depending on their placement can whittle the boss down to between 12 and 1/3 life remaining. If there are two of you with 8 a piece this can be a very fast fight. Honestly, if you run out of grenades and have plenty of battle gauge left, just suicide for another 3 grenades. I found that most weapons do little damage on extreme, or they have such poor accuracy hitting the moving head is near impossible. If worse comes to worse, aim at the 4 small side, and not the two big legs, as both stall the bosses, but the 4 legs come off much quicker.
2-3 boss - if you follow my strategy from hard it works just the same , have one person find the multi seat vs on the far side near where the train was. Just have the hold down the trigger as it has an unlimited ammo gatling gun. Aim for the leg, not the head. Hitting the leg does more damage and can prevent the most devastating leg fireball attack. Also to stand in the far corner from where you entered. Here you will be out of his range when he hops on the building, also he rarely comes to that corner. The other beauty of the vs is that it can jump and hover, rendering his wipe attack useless. Just jump as he rears back his leg, and tap the button to hover and he will miss. Plus the VS has high durability , one time he got me trapped in the corner and hit me a few times, but the VS lost on a little of its life. Your partner should get something like a rocket and take occasionally shots at the leg. IMPORTANT! When it fall down dead the first time, keep shooting its leg, it gets back up. But you can damage it while its down, and the damage is great , on hard and extreme you can whittle off about 2/3rds of its second life bar, and on normal and easy you can take out its whole bar.
3-1 - On extreme watch out, even if you leave the worm in the dust, it will still attack all the way up until 100 percent energy collected, and the last cart.
3-3 - get 2-4 people and divide up the tasks with at minimum a gunner and a loader/train cooler. But for the love of God watch out, even with a full bar when it says one last shot you have 15-30 seconds to fire off a round or you die. So when the life bar has only a sliver, make sure you have a full 3 rounds in the barrel to aim backwards and get the last shot. You simply dont have enough time to lad once you see that prompt.
Only recommendation on 4, if you are having a struggle with the four underwater VS area at the end of one of the chapters, have one of your guys make a run for the last data post, you can get it up without killing all the VSs. Easier to do with 1-2 VS left as one person can act the decoy.
5-2 boss - just a tip about the back, it has a secret area on the back that you hit and ball cannon like thing, but in reality it has two ball cannons and two secret back patches. You should be able to kill it on extreme with all 4 legs down, the first back patch and the ball canon gone, but if its still alive you have to jump past the ball canon thing further on its back. When it falls to the moving mech, have one man man the turret hitting the balls, and have the other make a dash for the back, grappling up the wing on the side and on its leg.
5-3 - There is a trick, dont go for the wings. Instead have one partner on the canon shoting the turrets on the side of the other vehicle, occasionally targeting the canon. The other person does the same but uses a turret. At some point ospreys come out , so focus on those. At some point the canon will start to smoke, and you will have hit all the turrets possible from your angle. At this point target one or two of the wings. Repeat the same strategy taking out the turrets and osprey. Eventually, you will do enough damage to the canon that it will be destroyed, even with one or two wings remaining. The benefit is with the turrets gone, ti relatively easy to make it to the other vehicle and get the data post. We encountered only one major attack on our post with the npcs doing adequate protection job. So once you survey the first attack on your own posts, have the other player character join the one who went to raid the enemy ship and clear out the room and finish taking the data post , after which you have a minute and a half to get back to your ship.
Honestly Episode 5 is the hardest in the game, if you make it past that extreme is as good as yours.
Some other general tips, snipers are accurate and kill in one shot, dont run and gun, always go for snipers first.
Have t eng conservation 1 and recovery 1 as your abilities.
I would pick gun sword, non energy rifle preferably gold, and rocket launcher as my weapons.
Last point, dont enter an area with many guards and no cover. If you get hit you cant aim, shot, throw grenades or heal. So if you get into an area with a lot of guys and no cover, they will kill you with out you being able to do anything.