Chris and Piers Chapter 5 has the BSAA attacking a super secret submarine lab. The final battle is at hand. In the lab and lobby, first you want to shut the switch off in Jake's and Sherry's chamber, then head for the spot where Chris and Piers separate. Chris will go under the deck, through the fan ducts. He will only face a few enemies, as opposed to Piers, who will face two gun turrets, RPG soldiers, and a lot of J'avo Spec Ops. Piers needs to shut down two fans, then head for the elevator to regroup with Chris. Enemies are infinite, so after using the two levers that control the ventilation fans, Piers may want to use a flash bang and leave.
Once you regroup, step through the next set of small labs and take out the J'avo. Enemies are thick, and your shotguns should work well here. Clear the area and move on. Chris and Piers will catch up to Jake and Sherry in the large Haos Chamber. Activate the switch on the panel to run power to the two elevators. When going up this particular room, you may combine the characters in different combinations for different pieces of dialogue (although game-wise, it's all the same: survive to the top).
When going up, doors in the Haos chamber will open, revealing enemies with marksman rifles and RPGs. Your lift will also have J'avo Spec Ops dropping in (two at a time) to attack you. Concentrate on keeping your selves alive (one player may deal with enemies on the lift, the other should snipe enemies on the side). When you get to the very top, Jake and Sherry will take a different path from Chris and Piers.
Escape the Haos Chamber
Chris and Piers need to escape the Haos Chamber before the platforms are destroyed. Climb the wall immediately after the cut-scene, and start running counter-clockwise (hold the sprint button). Apart from staying away from the Haos' hands, and keeping your speed up, there are few tricky spots in your way:
-Team effort: Vaulting up higher areas requires assistance from your partner. Piers is boosted up first, then the action needs to be used a second time to bring up Chris.
-Shoot down: Right past the first point where you boost-up, the Haos will try and attack you (the camera angle will change). Shoot the creature's head with high-damage firepower and it will be momentarily stunned so you may escape.
-Slide under: One of the first obstacles after the first pull-up ledges is a low obstacle that requires you to run and slide under the bar quickly. Use the shadow on ground to gauge your distance from the object. You want to slide quickly under it and continue. If you start crawling, you will probably die.
-Hang on: After the second boost-up and the second time you stun the Haos with weapons, Chris will need to boost up Piers, then hang on (QTE) while Piers stuns the Haos one final time so both men can escape on the lift.
Escape the chamber, then fight the Haos by focus firing on the monster's head. Ammunition around the arena will fill your stocks back up (you may want to take them first, since defeating the Haos forces you to leave the room). The creature is rather slow, and has few attacks if you hang at the edge of the ring and use rifles and handguns.
Shooting the Haos may move you to the next sequence, but story-wise, it proves to be a waste of ammunition. Stock up on the grenade rounds and shotgun shells. You will need a powerful weapon to drive off the Haos' hand when it blocks your escape. The Grenade Launcher's explosive rounds are perfect for this, as one explosive shot will probably hold the hand at bay long enough for you to run under and past it. Letting the blast doors close on you is a bad idea, as is taking too long to make your escape from the tunnel section (you die). Sprint forward and wait a bit at each blast door (to make sure the hand doesn't intrude in your path). At the last blast door in the first tunnel section, Piers needs to spam a button to hold the door open while Chris needs to climb up as quick as he can to escape.
Escape the Haos in the second part of the tunnel and you will be treated to a cut-scene. Here Chris will do nothing, but Piers (player or bot) will need to crawl to the syringe and use it. Piers will not use his weapons anymore, nor will he have access to his health pill Pez or his inventory. He will naturally recover life, since his bio-electrical shots and charge-up shot will use up his vitality. Chris in the meantime, will fall back to his stand-by weapons.
Defeating the Haos is about piercing the two glowing blisters on its waist. Once both nodules are popped, the boss enters a cocoon. You may attack this cocoon (before you are hinted to do so by your A.I. partner) and should you break the Haos out of the cocoon prematurely, you can go up to it and initiate a melee QTE. Complete the melee QTE in order to damage it. Repeating this process enough will cause a cut-scene to occur, where Piers will stab the Haos, and Chris will need to crawl up to the boss and do his own QTE to stab it. Successful completion of the QTEs by both players will kill the Haos once and for all.
Piers will retain his B.O.W. arm and will need to use it to escape the final section with Chris. Since the section is timed, don't bother fighting all the creatures in the collapsing walkway. Piers needs time to charge and blast the organic roadblocks in his way (he should stand close to, or next to them) since the range of the charged shot is very short. Chris in the mean time, should follow and keep enemies off Piers so he can do his thing. Reach the end (there are three organic barricades) and you will have completed the final chapter for Chris and Piers.