For Flood Mode, all you need to do is open your conf.ini file and paste this inside:
Here's how to get it the authentic way (no conf.ini editing), it also gives you the achievements
Try to get a spawn next to wall, or in the corner (better option), you must be on normal or above
Get all 3 slush traps, and place a beacon as soon as you step into 1 (which is why I want you to have a spawn next to a wall or corner)
After getting all 3 slush traps, you then have to pick up the gun, NOTE: You must not pick up the gun beforehand.
Use your super weapon to kill the 3 little creatures that are colored yellow, they spawn where you activated the slush traps
After doing all of the above, finish the game like normal, hopefully you spawned next to a wall or in a corner.
Now enjoy Flood Mode!