Turbine

Level 2-2 takes place in a large, circular ventilation shaft, and is notable for never changing its layout, but introducing tougher enemies such as Hunter and Mega Kegs in slightly larger numbers. Bullet Turrets also spawn in the center of the map, giving the player plenty of room to see how they operate. While you could rely on your primary weapon to get you through Descent easily enough, learning how and when to use your secondary weapon and the close space is the key to victory.

Waves
Wave 1 (8 enemies): 4 Hunter Kegs crawl out from the holes surrounding the center. 4 more spawn as the player kills them.

Wave 2 (32 enemies): 5 Kegs and 1 Mega Keg crawl out, with 25 more Kegs and 1 additional Mega Keg spawning as the player kills them for a total of 32.

Wave 3 (12 enemies): 6 drones fly out of the holes, 6 more spawning as the player kills them.

Wave 4 (32 enemies): 6 Kegs crawl out, and 1 Bullet Turret falls into the center. 24 more kegs spawn as the player kills them for a total of 30. 1 additional Bullet Turret falls after the player kills the first for a total of 2.

Wave 5 (32 Enemies): 20 kegs crawl out from the nearest hole to the player, with 12 more spawning after the player kills them.

Wave 6 (24 Enemies): 8 drones fly out of the holes, with 16 more spawning as the player kills them for a total of 24.

Wave 7 (36 Enemies): 4 Kegs, 4 Drones, and 1 Mega Keg crawl out of the holes, and 1 bullet turret falls into the center. 24 more kegs, 1 more Mega Keg, and1 more bullet turret spawn as the player kills them.

There are 176 enemies in total.

Strategy
While there are holes in the level that the player can't cross, they're completely safe and serve as on obstacle for movement. Use these to your advantage by separating yourself from the large amount of Kegs that spawn in waves 2 and 4, 5, and 7. The close quarters is the only thing making these Kegs dangerous, but an Accelerate powerup will make it easier to keep away from them.

Area of Effect Assault Androids like Coral and Aubergine can hold the center of the stage rather well, and collecting power-ups is generally easy for them. Other Androids should make use of the circle surrounding the center to strafe around and pick off the enemies from a safer distance. Lemon also has an easy time using this tactic, as her upgraded primary weapon can cover most of the stage from any place on the perimeter.

Developer Commentary (paraphrased)
T: So this is Turbine, and it's based on one of the earliest prototype levels, actually. One of the first layouts I built was this round level with a couple of paths out of the main area. It was all based around trying to create chokepoints.

S: I think you had a giant fan at one point, didn't you?

T: Yeah, that's why it's still called Turbine, actually, even though the fan is now missing.

S: It plays a lot better at this scale, though. I think this is about half the size it originally was, which was just a little bit too overwhelming for people, and also you ended up being away from the enemies quite a bit.

T: I also like how there's a "Caution" sticker on the ground, even though you can't fall off the edges.

S: Well, you have to be cautious. Gotta give people that scare. I always enjoyed the aspect of this level the most, where there's just so many enemies at some point that it becomes absurd, but it's still fairly easy to deal with. It was a really great moment for me because people would freak out when they saw it happen - they would get surrounded, they would get mauled, but then they would defeat everything. It kind of works in line with the theme of the entire game, which is this sense of doing the impossible and overcoming ridiculous odds.

T: This is a level that has a nice sense of teaching you. So when you first play it, you're taught to get out of the middle of the level when all the enemies drop on you, but later you learn how to deal with it. So you come back to it with "veteran eyes" and know how to better handle threats.

J: Well, the game ultimately rewards you for playing more aggressively, doesn't it?

T: Mm.

S: Yeah, it's a central concept to the entire experience. It's very important to nail it down early on. It's also just... a simple level, I guess? Which is really nice at this early point throughout the game.