Getting to the Blue Key Slot (10)

First off, keep an eye out for a couple of secrets. Just opposite the candy machine in the current room, you'll find a bookshelf. Interact with it, and it'll swing open to reveal two Devastator Ammo pickups. Grab those – they'll be super useful.

There's another secret near where you're headed for the Blue Key. Look for a section of a door that has a crack in it. Blast it open, and you'll find a hidden room. Inside, you'll find several Slimer Eggs, a Chaingun Cannon, and some Freezer Ammo. Stock up!

Now, head back to the Blue Key door. You'll notice two more Battlelords have spawned on your path back. You can either take them head-on or try to sneak past them if you're low on health or ammo. Your call.

Once you reach the Blue Key door, open it up. Inside, you'll find an elevator with a Pig Cop waiting for you. Take the elevator up.

When you arrive upstairs, you'll be in an area with conveyor belts and some Enforcers ready to attack. Immediately to your right, as you enter, there's a switch that deactivates the conveyor belts. Right in front of this switch, you'll find Chaingun Cannon Ammo. Grab it before you hit the switch.

After deactivating the conveyors, proceed deeper into the room. You'll encounter two more Enforcers that you'll need to fight. After dealing with them, you'll find two RPG Ammo pickups.

If you decide to follow the conveyor belt downwards (after you've dealt with the enemies and picked up the RPG Ammo), you'll end up in an area where you can find an RPG. There are two switches near this conveyor belt. One switch closes the door leading to the RPG, so be careful which one you hit. The other switch causes a compressor to come down. Make sure you grab the RPG before you mess with those switches too much!