Before anything, I'll have to apologize for the occasional stutters and apparent framedrops in these runs. There is little to be done about this; Alien Swarm isn't terribly well optimized, and it's also forcibly played through Steam servers. Most spikes are actually just client-server desync, rather than performance hits, and thus are completely unavoidable.

It's worth keeping in mind that this game is designed as a 4-player coop game. I complete every level on the hardest difficulty, solo. Without wasting any time. Furthermore, all but one level reach the in-game speedrun achievement time, which are also designed for 4-player teams. That should contextualize the difficulty somewhat.

Most levels are completed using Wildcat, because she is the fastest (highest movement speed, high damage). The only missions she's not used in require a tech to hack a main mission objective. For these, both techs have an advantage: Crash has higher explosive damage, while Vegas has higher melee (thus chainsaw) damage and sets up turrets faster.


Individual level notes:

Landing Bay (1:17)
Despite being the first level, this is the hardest one to beat normally on Brutal; it simply throws so many different obstacles at you that a single player can hardly tackle it. Luckily, this level fully allows OOB movement, so I knock a barrel close to a fence that isn't blocked by an invisible wall, and just roll over it. Stimpacks help speed up the hacking times, since the run uses an in-game timer, and doing the last hack fast enough to survive the alien swarm without one would be impossible anyway.
This run isn't perfect, but the barrel physics is highly inconsistent and the hacks are completely RNG, which makes improving this run very difficult as is.

Cargo Elevator (2:06)
The first half of this run is honest speedrunning: perfect movement and accuracy is required to burn a hole through the swarm (and plenty luck to grind out). The tiny edge allowing me to walk over the pit is well known in the community.
The hard part is the elevator, but not for reasons you'd expect. Rather than having to survive that gauntlet, I simply stand in a location where an alien will eventually spawn. Overlapping hitboxes forces my character on top of the alien, allowing me to walk over the fence and roll down to the elevator shaft. This boost happens at a random time, and it needs to be as fast as possible to avoid aggroing the fatty by the doors. Then, eventually, the game realizes I'm not supposed to be up there, and it puts me on the closest pathable area, being the other side of the wall.

Deima Surface Bridge (2:13)
With those silly OOB routes out of the way, it's time for the other fun part of Alien Swarm speedrunning: Assault jets, aka rocket boots. These items are surprisingly well coded to prevent sequence breaks: it's always impossible to jump to an area behind a mission objective, or even just a natural barrier. Still, they can cover large distances with precise aiming and save a lot of time.
This is one of the easier levels, the only real risk of death is the large swarm of aliens at the end, but their spawn location is very consistent so it simply takes practice.

Rydberg Reactor (3:10)
Reactor might be the most difficult level to complete. The mission consists of three different gauntlets, one of which requires a lengthy hack, the other is positioned in a tight corridor. The hack can only be successfully completed under the protection of a flame turret, and then random alien spawns will determine whether your hack is successful or not. And then you're forced to kite aliens around for half a minute while the door opens slowly.
The second door can only be survived by legging it. A huge swarm is on your heels and the flame thrower can neither kill nor control them long enough. Luckily, the lap around the level I run takes just as long as the door opening, so while it may look comical, it's safe and doesn't waste time.
Then the final door is simply a game of touhou; constant barrages of projectiles flying at your head, and you dodge them until you win.

SynTek Residential (2:52)
If Reactor is not the most difficult level, then Residential is. This level is highly inconsistent due to unpredictable spawns and many possible sources of death. It's a very tight level to execute, but random deaths happen constantly.
The most inconsistent part is the first intersection. The fastest route is to move up to the first biomass, put it on fire, then backtrack. But usually this route causes parasites and swarmlings to come out of the vents and destroy you. Landmines are used specifically to protect against them, but that still doesn't reach full consistency.
The off-screen shots towards the north kill three swarm eggs that sit behind a door. The marksman rifle pierces doors, so it's the fastest way to destroy them. To speed this up just slightly, I throw landmines to reset the gun's refire rate, a pretty odd but neat glitch.
The rest of the level, especially in the vents, consists of the most efficient movement, well practiced shots, not getting infected, and lastly another difficult gauntlet while waiting for the elevator.

Sewer Junction B5 (1:09)
Another rather straight-forward level, made difficult by two tricks. The first is an off-screen rifle shot that blows up a barrel and clears an obstacle; usually you'd have to wait for its animation to finish. It's an extremely inconsistent shot as you have to shoot it while running down the stair; you can't reach the barrel from the top, and you won't shoot upward if you're below. I don't have a consistent reference point, it's pure grinding.
The rest of the level is dodging parasites (they can't infect you while the charged armour is active), blowing up all the eggs, and finally a small shortcut in rolling across the gap.

Timor Station (3:33)
This level suffers the most from the limitations of the assault jets. The two jumps to cross the bridge early are very useful, but its other uses aren't nearly as significant, sadly.
I can't afford to take any protection for the hack, so I simply have to clear the area of all parasites, clear two waves of aliens, then get the hack quickly and leg it. It's not too inconsistent, but parasites will be parasites.
The easiest way to lose time on this level is the fourth jet boost. For some inexplicable reason, the game is extremely picky on how you're allowed to jump over the rocks. It's entirely mandatory as there's no way to safely move past that swarm of aliens, but it often ends up being spamming the button for up to 5 seconds praying the game recognizes the right angle. Miraculously, this run got the jump instantly.
The rest of the escape isn't entirely smooth (shoutout to the shield bug boost that almost saved time, but didn't), but it's acceptable considering the rest of the run.