After going on lower level, you'll find yourself in the complex, which is controlled by your allies. Gather some ammunition and find engineer Strauss from the Rhino Squad (#31) - he'll renew your armor. There're door right behind him, so walk through them. Move straight ahead. If necessary, visit also the medic.
You'll run into Morris. Return with him to the trenches, where the commander Voss is waiting (#32). In reward for your successful efforts, he'll give you Grenade Launcher.
Capture the Air Defense Cannon
Practice launching grenades in the trenches, which are full of enemies. You should get near the cannon, but the door leading to the bunker will be closed (#33). Voss will order you to get inside somehow. Jumping over the chests (#34), reach the top. You should see small opening (#35). Go inside and push the button. The door will open and the rest of the team will join you.
Run with others and kill encountered enemies. In the big hall (#36), jump over the barrier and send some grenades to opponents standing on the balcony (remember that Voss has HyperBlaster, so don't try so much). Few seconds later, the door on the ground level will open and you'll fight with another pack of enemies (of course use grenades).
Move straight ahead and kill more opponents. Now, you'll enter big cannon's control room and use it against the Stroggos.
Sit in armchair (#37) and wait, until the sight activates. Shoot in giant hatch (#38) and leave the place.
Locate the MCC Landing Zone
Return to the door, but don't go further. Instead, use elevator, but watch out for the enemy (#39). On the balcony turn left, kill some Stroggos and run the only available path. Use panel to open the door and go to the trenches. Get out from there (#40) and observe the cannon's destruction.
Soldier will run into the destroyed building. Move with him and eliminate some enemies on the way. After short walk, you'll find yourself outside (again).
There is a Stroggos' building located on the right side (#41), so be careful. Stay close to the wall and walk forwards. Few minutes later, the giant (allied) spaceship will arrive and eliminate any danger.
Board the Hannibal
Wait until the spaceship will land on Strogg's planet surface (#42). Run near it. When the platform lowers, walk inside to finish the level.