Umissable story achievement. This is the second longest act; longer than the second but shorter than the first.
Here's a list of the chapters so you can get a feel for where you are:
- Arrival at Channels
- In the Floodgate
- Crab Claws
- Death from Above
- Death from Below
- Blood Under the Bridge
The only points that may give you trouble are the crab bosses in Arrival at Channels and Crab Claws. These guys are pretty unbalanced and unfair, but also pretty stupid!
What I did for Channels crab was hide in the open container very close to where you descend. Just pop out and put rounds into his legs whenever he stops shooting, then go back into the container to avoid grenades. Just repeat for like 20 minutes of boredom until he finally drops.
For Claws crab, I just circled around a pillar (to the right from where you enter) and let him fill the pillar with lead, then I popped out and returned the favor when he was cooling down. Again, repeat for what feels like forever until he goes down.
Don't need to mess with rocket launchers for either as that will probably just make it harder and more frustrating.