Halo: Combat Evolved Anniversary

I found a video that actually helps past the flood point, they say which routes to take, you get the achievement when you activate the last lift.

If you have it, use the bandanna skull, but don't use double explosions because of the flood carriers, also use a shotgun and frag grenades when fighting the flood.
You can also earn this achievement in the same level.


- Credit goes to roosterteeth for the video.

Here is how to get this achievement without using any grenade jumps and without making that difficult column jump. No video is required, just follow these directions. After each step, you can pause the game to stop the timer and review what your next step will be. The 21 minutes starts when you enter the facility, (where the first elevator is):

Part 1: To the Flood room:
1. Take the ELEVATOR down. At the bottom of the elevator, move forward under the awning. There will be several enemies to kill. Just toss a grenade in to clear them out.
2. In the next room drop DOWN to the lower level and take the door to the RIGHT.
3. In the next room, take the door to the LEFT.
4. In the next room, take the door to the RIGHT. There is a soldier sitting there who shoots at you. This room has a place where you can climb up to the upper level. Climb past the fire to the upper level and take the door on the RIGHT. It should be the only door open (green).
5. In the next room, you are now on the second floor. Turn on the LIGHT BRIDGE and go straight across, through the door and around the corner there will be RAMP heading down. Go down the ramp and into the room. There will be a CUT SCENE and this room will have closed doors where Flood will break out. When each door busts open with Flood, toss a grenade to clear most of them.

Part 2: To the DOWN ELEVATOR
6. When you get the cue to Escape, head out the door you came in and head RIGHT up the ramp and LEFT around the corner. Go through the now open door.
7. In the next room, stay on the top floor and take the door on the RIGHT.
8. In the next room, take the door on the RIGHT again.
9. In the next room, take the door on the RIGHT again, which leads to a long hallway. Go down the hallway, turn RIGHT, then there will be a door on the LEFT. Go through this door and around the corner there will be another ELEVATOR. Take this elevator down.

Part 3: To the UP ELEVATOR
10. At the bottom of the elevator ride, go straight under the awning. The next room will have a light bridge that is flickering on and off. This is the room where video guides show to make a difficult column jump to the door on the right. This guide assumes you have failed to make that column jump and have fallen down to the lower level. On the lower level, take the door on the LEFT.
11. The next room is a room where you can easily jump up onto a toppled column and get up to the second floor again. Once on the upper level, take the only open door at the front (back) of the room.
12. In the next room, you want to go straight to the door directly across the room from where you entered, past the LIGHT BRIDGE on the right.
13. In the next room, you will need to drop DOWN to the lower level and take the door on the LEFT.
14. In the next room, you will want to head straight across the room from where you entered, past some flood standing in the ditch in the middle. Go through the door that is behind a column directly across from where you entered the room.
15. In the next room, there is a place where you can climb (jump) up to the second floor. Climb up and take the door on the LEFT.
16. In the next room, turn on the LIGHT BRIDGE and go across it. Go straight through the door and around the corner there will be an ELEVATOR to take you up and out of the facility.
17. Head out of the facility and the achievement will pop. If you have avoided as much combat as possible inside the facility, you will finish the level in ample time to earn the achievement.

This is perhaps the EASIEST timed achievement I've ever gotten. The strategy is very simple, as The Doctor would say "Basically... RUN!" that's really all you have to do. Set the game on Easy, put a shotgun in your hands, keep it loaded just in case (or turn Bandana on), and run through the map from the first room where you encounter the flood (after the long cutscene with Jenkins) all the way to activate the final elevator. The hardest part of this is potentially getting turned around but if you've played the game as many times as most people have then that shouldn't be a problem or an easy problem to fix. I timed myself and it took me only 7 minutes, 1 minute or so in the first room then 1 minute or so I got turned around but 21 minutes is TONS of time if you don't just try to kill everything that moves. Only shoot things that get in your way or that are shooting you too much but no need to bother on anything else.