Serious Sam: Siberian Mayhem

This achievement, as of 4/18/2024, is FIXED!

You can do this in co-op by yourself, and you should, as it only unlocks for the host. It is easily done by going to Multiplayer and in Custom options, set the enemy multiplier to x2, you DO NOT have to alter the enemy strength percentages. You can put on unlimited ammo, and give yourself unlimited respawns, as well as temporary invulnerability. Having unlimited ammo and gadgets mean you'll eventually complete this just ramming through it.

Some tips:
-If you're impatient like me, you can spam the Serious Time and Serious Rage gadgets and run past enemy spawns, and then spam things like the Companion Drone, Black Hole, and Nuke on parts where you are forced to kill enemies. As the game is only 5 missions long, this makes the game about 90 minutes. This does slow down your game and could possibly cause a crash, so be warned. Swap your game to PERFORMANCE mode to decrease the chances of crashes, as this game doesn't run very well on console (even on Series X).
-Has to be done in one go. If you disconnect or crash, it must be redone.
-Recommended as a second playthrough so you know where the better gadgets and weapons are, and you can endlessly use these with unlimited ammo!
-Mission 4 has an issue at the end train section where if you kill enemies too quickly it'll just have you sit there with no more enemies to kill. THIS IS NORMAL. I had this happen on both the console and PC version, and sitting there for a few minutes will eventually force the game to continue (you'll hear dialogue and be able to board the train).

Old Solution (if someone is randomly curious to read how long this took to get fixed lol):