Beat all the challenges!
TheRedRodCod has done a great job of writing out the challenge descriptions in his solution, and there are equally as helpful solutions for the SPEEDRUN and SPEEDRUNrt challenges under those respective achievements, but this solution will focus primarily on the NORESTART challenge.
Before that, it's helpful to know that you DO NOT need to achieve the Red Times for the HOTSWITCH challenge, just finish them. On to the NORESTART challenge:
* Death is not allowed.
* Complete the game in one run
* ...but time stops completely when you stop
The biggest tips I can give are all variations on the fact that time doesn't move when you look around, so look around
. Know where everyone is at, plan your next move. Also, looking to the side will allow you to dodge bullets as they come at you; good rule of thumb is if you can follow the bullet by looking directly at it the whole time then it will miss you 90% of the time.
And don't be afraid to cheese it. Hide in rooms, doorways, and/or corners, walk into the wall to advance the enemies if you need to until you can cheap shot them from around the corner or through the wall sometimes. Jumping will
get the enemies to fire wildly but be careful. Having said that it is
my preferred way for getting to safety at the beginning of the last level (check the video), and it is easily reproducible.
Anything else is just basic pointers on playing the game well, so I'd definitely do this challenge after all the others so you know the levels, enemies, and tricks backwards and forwards. I hate it when people say this but: altogether it's not too bad, here's a video of the stream I did when I finally pulled it off (successful run starts at the 1 hour mark):