Serious Sam HD: The Second Encounter

There is a console command that will allow you to easily set valid survival times in both single player and co-op:

phy_iMaxThreads=0

Opening the console (default: ~) and entering this line will prevent all players, enemies, and projectiles from moving. The timer will continue to run, so this makes it possible to achieve gold medals by idling. Note that all waves of enemies will continue to spawn which will cause a lot of lag.

Changing this value back to its default value (16) will likely cause the game to crash if too many enemies have spawned. The best way to end the round is to damage yourself with explosive weapons. So, this strategy may not work on Kleer Base since no explosive weapons are available. For Kleer Base, there is an effective safe spot strategy that can be used instead: https://steamcommunity.com/sharedfiles/filedetails/?id=479525747

Time (In minutes) for Gold Medals:

1. Bear City: (Co-op) 09:00 | (Singleplayer) 5:00
2. Bend on Sand: (Co-op) 5:00 | (Singleplayer) 3:00
3. Death From Above: (Co-op) 7:00 | (Singleplayer) 3:30
4. Deep Hole: (Co-op) 6:00 | (Singleplayer) 3:00
5. Green Field: (Co-op) 10:00 | (Singleplayer) 8:00
6. Icelander: (Co-op) 6:00 | (Singleplayer) 3:30
7. Kleer Base: (Co-op) 6:00 | (Singleplayer) 6:00

You can disable Serious Sam HD: The Second Encounter DLC named "The Legend of The Beast" so you don't need to complete optional level for those achievements

**Optional ** 8. Tripple Trouble (Co-op) 9:00 | (Singleplayer) 5:00

You can rocket jump up to the wall from the left side where you spawn
After that walk around entire map and rocket jumpto Cathedral.