First before you start the game, you should choose your character. Here is a list of all characters:

• Claudette - good.
  
• Dwight - also good, but have problems with healing yourself.
  
• Feng Min - the best.

To get Feng Min you should complete 1 run on any difficulty as in the screenshot below. It is not hard since you can do this on very easy difficulty, and it's completely not a problem to get her.

After you start the game, you appear in a room full of monsters.

• At 1-3, 6-8, 11-13 stages you should kill all the enemies and repair gens (do it as fast as possible 'cause enemies get stronger with time).
  
• 4,9,14 stages are just safe zones, where you can just heal yourself by standing on the lake, but don't spend lots of time there (described in brackets above).
  
• 5,10,15 stages are the hardest. At them you should ignore usual enemies if it's possible and concentrate on the bosses.

For the first time I beat this game in 81 minutes solo (showed in the screenshot below). It was suffering because I was fighting with the monsters for too long. That's why don't waste too much time at one stage and move on as quickly as possible!