Chapter III: Disturbing Revelations
Once you get past the lake and up to the next checkpoint, the game's first Kantus will send another Locust swarm your way. Use some sniper fire to take out the big boss, since he's easily the most annoying Locust in the bunch. Once you're past the doorway, grab the Locust Emblem on the ground to the right.
There's a fork in the road further into the temple, so choose a path and gear up.
[Shoot the glowing fruit to lure the Rockworm.]
If you take the left path, use the Rockworm to give your teammates some cover, but watch out for the swarm of Wretch grubs that will storm the room. Once Dom and Carmine are in the clear, flip the lever and head down the door to the left. If you took the low road, once you're through the aforementioned area, provide some cover fire for your teammate.
The Locust defenses may be toast, but that freakishly huge earthworm will make another appearance at the end of the chapter, sinking an entire city to your team's dismay.
Chapter IV: That Sinking Feeling
[Investigate the King Raven crash site.]
Just after you witness the collapsed building, you'll have to deal with some Bloodmounts, so have grenades and heavy artillery handy. When that's over, a Reaver will crash into view, so get behind some cover to avoid getting crushed. Shoot off the pilots if possible, but try your best to keep the grub from launching explosives.
After you lose radio contact with Cole, a Corpser will signal a new wave of Locust swarming out of the far tunnel. Find some ammo and fire away, but make sure to fall back when the next Reaver lands. Sniper fire will definitely work best here, but just make sure to work with Dom and Carmine to create some punishing crossfire.
When you get to the crash site, the King Raven's crew is obviously in no condition to be rescued. But, a fresh wave of Locust will be more than happy to ambush your team. There's no good cover out in the open, so head back into the archway and try to take the best shots you can. Eventually, the Cole Train himself will come busting through the scene, saving your hides and doing it with embarrassing style.
[Search for Baird and Sigma Squad.]
Scout the area for a corpse with a COG tag, and pocket it for your War Journal.
At the next checkpoint, a Reaver will attack you in the graveyard, followed by Drones and Tickers. Kill the Kantus first before it can heal the other Locust, or you'll never make it past the bridge. Once you're done here, do an ammo sweep and move on to the next chapter.
Chapter V: Captivity
[Search the area for other captives.]
For a gear, Baird is one whiny guy. Once you rescue him from his holding tank, head deeper into the area, but make sure to hang a right at the large open cavern. There's another collectible lying on the ground in front of a torch for your War Journal.
[Find a way to get the barge moving.]
Continue on and you'll come across a Beast Barge. Board that sucker and kill anything that's ugly. If there's any ammo left in it, snag the dropped Mulcher and head up to the top level. Once you're done handling the Locust up top, do another ammo sweep and hit the switch on the center column.
Make to man the nearby turret or shoulder that Mulcher in case any Reavers try to ruin your fun.
[Board and search the adjacent barge.]
While your Mulcher will disappear, you should still have plenty of ammo for the next attack. After you and Dom handle the Locust on the other barge, hit the switch in the middle and you'll finally find Tai. After the brief cut scene, you'll hit the next checkpoint and complete the current objective. Get off the barge and storm up the bridge. Keep an eye out for the Grinder and his Mauler; try using sniper fire to take him out from the left side of the bridge.
[Find a suitable location to contact Control.]
Head down the slope to the right after a quick ammo sweep. If you happen to have a Mauler handy, set up on one of the cover points and use that baby to chew through the incoming Locust group. Anya will contact you with a new objective when you're through squashing grubs.
[Move to the extraction point.]
Run through the ruins to get closer to your landing zone, but be careful about which branching paths you take. At the first point where the fork in the road meets up, a Reaver will spot you. If you're out of Mauler shots, use the biggest gun you've got.
There are plenty of manageable Locust groups hiding out in different areas, so use the terrain to your advantage. In some cases, if your team goes down one route, you'll be able to double back and catch some grubs in a crossfire. When you get close to the only (mostly) intact building, head up to the roof after doing a thorough ammo sweep.
[Defend the King Raven extraction point.]
Aren't you glad you did that ammo sweep a few minutes ago?
Your position is going to get hit hard by tons of Boomers and Drones, so stay as close to your teammates as possible. Try to hole up and defend yourself in the rear area of the roof, and pick off bold Locust from doorways and cover spots. Eventually, you'll notice a timer showing how long you need to defend yourself until KR One-Nine comes in to save your bacon.
If you survive, you and your team will reach the chopper in time... but not before that monstrous, city-sinking worm decides to crash the party.
Chapter VI: Intestinal Fortitude
[Find the Riftworm heart.]
Oh, okay, so it's apparently called a "Riftworm". Well, either way, you're going to have to venture deep into its body and kill it. Once you have control of Marcus again, look for a magazine to the left of your starting point and add it to your journal collection.
[Avoid the digestive teeth.]
It goes without saying, but this would be a horrible way to die. If possible, abuse the Roadie Run when appropriate and rely on your teammates to show you the proper timing. Eventually, you'll run into Carmine and a swarm of nasty little crawlies, but it looks you're too late to do anything to help the poor rookie.
[Outrun the wall of debris.]
The wall doesn't move too quickly, but you'll still have to book it Indiana Jones-style, so run, run, and run. Pick up the COG tag on the left side on your way out, just before you hit the first turn. If flesh walls and membrane sheets are holding you up, just use your chainsaw to open up the paths. While you're running around, make sure to avoid the protruding spines altogether. Once you're past a few barriers, you'll be home free and the wall of debris will crash safely behind you.
[Avoid jets and pools of acid.]
If it's green, it's definitely mean. Shoot the acid nozzles to stop them from spraying the path with their alien death juices and Roadie Run to safe zones. At the starting point, take a brief detour to the right if your rifle is running low on ammo.
[Pass through the intestines quickly to avoid suffocation.]
If you head in a relatively straight line, you'll eventually make your way to next membrane wall. Rev up your chainsaw and cut through the next membrane barrier to get through to the next checkpoint. Be careful; there are plenty of dead ends in here to get you lost.
[Cut two arteries to destroy the Riftworm heart.]
Use your chainsaw on the arteries, and a path will open up as the Riftworm's blood begins to spill. Watch out for more of those crawlies that tore apart poor Carmine. Once you hear the Riftworm's heartbeat speed up, you'll need to search for the rest of its hearts and continue your elective surgery exercise.
[Cut three arteries to destroy the Riftworm heart.]
When you start hacking away at the first artery, the room will start to fill with blood. If you take too long to cut the rest of the tubes away, you and your team will drown in worm plasma. Cut everything away as quickly as you can, and a path will open up to the last Riftworm heart.
[Cut four arteries to destroy the Riftworm heart.]
You can exterminate the intestinal Locust before hacking away at the last heart, so use up any ammo you've been holding back. Once again, the room will fill up with worm blood as soon as you cut the first artery, so don't get stuck in the blood pool. As soon as the final cord is cut, Marcus and the rest of Delta will make a successful, but very undignified, victory exit.