1. From the start, while only one tunnel is overrun with Antlions, grab the Machine Gun from one of the tunnels and take it to any other that needs to be defended. By placing a turret next to two rebels, you won't have to worry about breaking the blockade or running out of ammo.
2. When two tunnels are under siege, I place a Machine Gun in the red-lit tunnel #1, then run to another Machine Gun and move it to the burning spot in tunnel #2. Rebels always go only to their designated locations.
3. When three tunnels are under attack, I place Machine Guns in the ones that aren't defended by rebels and are nearby.
4. When four tunnels are about to be attacked, I quickly position Machine Guns on the undefended walkways and try to position myself at #24 or the other side to be near the shotgun or machine gun ammo crates, then hold the position until Worth starts yelling that he's under attack. After stocking up on ammo, I run down the ramp from the nearest tunnel. I destroy everything that flies over the railing or runs down the ramp. When I run out of ammo, I alternate between the shotgun and the submachine gun, continue firing, and start moving up the ramp to support the rebels and replenish my machine gun ammo from the crate.
After three lights appear in tunnel #12, three Vortigaunts arrive, and I remain at the base of the ramp, using the walls to minimize the attack area. If I use the SMG, its ammo burns through pretty quickly. Then I switch to the shotgun, and if I run out of supplies, I'll have to run back to the crates in the chaos. Try to find the right moment, because now you have two rebels and three Vortigaunts fighting for you.
Note that Spiders always easily avoid ranged attacks behind boulders. If you're at a distance and can't aim, try stepping to the side and unleashing a burst of automatic fire. This increases damage to monsters and keeps them at bay. Scale as needed to try to improve accuracy and minimize personal damage.
Follow the Vortigaunt off the ramp, past the elevator gates into the tunnel; destroy the boulder with the cannon. Go through web #1 and through the side web for shotgun shells and/or grubs. Here you should hear "Grim Peanut." Break the web blocking the path - this is where the action resumes. If you watch, you can see a spider crossing the tunnel from deep within the next exit. Continue toward the grubs on the ground, stop and remove web #2, cross and stand directly in front of it to spot enemies deep near a small stalagmite with grubs. Grab a grenade and throw it into the cave ceiling above the spider.
You'll need to maneuver quickly to avoid additional enemies in the cave. Once you duck through the crack and find yourself in the next tunnel, you'll usually be safe and can move on to the next web for machine gun ammo.
Use the right side of the exit tunnel as cover from the Spiders approaching from the far right tunnel; let the Vortigaunt zap them. Reach for the revolver after killing the two white monsters in the distant tunnel to the right, and switch to the shotgun. Once outside the cave, find the rising ledge/path leading to the cave to the right of the exit (the distant tunnel) and run and jump straight into the crack.
This maneuver can eliminate two or more enemies until the Vortigaunt is defeated. While firing at the monsters with your machine gun, continue toward the side lane for machine gun ammo. There might be three ants and a couple of spiders waiting at the next bend ahead. I grab my grenades, move to the leading edge of the next section, and throw two into the gaps in the tunnel. Then retreat and switch to the shotgun. If no one shows up at the bend, fire a burst of machine gun fire, and the spiders will appear more often. The vort is usually close by and will shock the monsters crossing the bend.
Find and drop into the hole at the end of the supply tunnel, then run to the blocked gate, remove the planks with your cannon, and quickly enter the room filled with zombies. Run through it to the wall and blow up the monsters. Find supplies, grab a barrel, enter the next room, place the barrels on the elevator, and place heavy steel objects on it. Head down the shaft as quickly as possible to slam the cage door on the zombie and grab the barrel before the monster tries to get out.
Elevator #2 has a control panel and a zombie next to it; Collect 3 Steel Bundles near and below the elevator, then place them on the lift. After killing the zombies, activate the button on the panel and return to the room. I throw 2 Steel Bundles; I leave the 3rd and start climbing. As you rise above the level of the top floor, find a gathering of zombies. Raise the gate for the Vortigaunt, go to the exit point and wait for it to power the generator. On the supply mine cart, jump to the end closest to the pit, then smash the boxes and grab the ammo before it flies down.