Operation Burn Water

Once you've done all previous side missions, continue with the main story by selecting this mission.

Follow your team on the planet until you reach the base. Drop down and wait with entering the main door (closest to you), instead going a little further to the left to reach an optional armory with an equipment upgrade, a Banshee Weapon, and (sometimes) the KBS Longbow Weapon.

Armory, Banshee and KBS Longbow Weapons

The next part of the mission consists of you, Ethan and a third teammember sneaking/killing your way across the linear path up ahead. You'll need to take out (groups of) enemies. Your friends will clear out the ones you don't target. Continue until you reach a ventilation shaft. Crawl through.

In the area up ahead, go downstairs, collect some Seekers and throw them in the open. Clear any remaining soldiers to continue. A C12 battle tank is dropped. Follow it (just don't walk in front of it or you'll get hurt by its heavy steps) until you reach an open area with a lot of enemies. Press R2 and use the C12's rockets to wipe out most enemy troops up ahead.

The next area is full of enemy troops, so clear your way through them, using Seekers and regular gunfire to do so. At the end of the area you'll face an enemy mech. Fortunately for you there are bazooka's scattered all over the place (and indicated with the proper markers). Or, excuse me, I should call them Spartan SA3. Destroy the mech, continue and quickly move to the left side of the path to reach the door up ahead. Simply ride the elevator to the top once you're inside.

Spartan SA3 Weapon

Hop inside your Jackal, clear all targets and hop out again. In this area, simply push onward on the linear path until you reach a switch; flip it, return to your Jackal and fly to the tower, then destroy it to conclude the mission.