I give no secrets.
There are three criteria for unlocking this achievement:1. All bobs, aside from evil bobs, have to be dead or have teleported out before you end a level2. Every switch that you use needs to be either A) Punched with fist, B) Shot with Assault Rifle's Grenade, or C) Shot with fusion pistol. DO NOT USE/ACTIVE THEM NORMALY, NEVER EVER. To press is not to punch.
3. The difficulty must be set to "Total Carnage".There are also three different ways that you can go after this achievement, each with its own pros and cons.1. You can simply start a single player game, set the difficulty to "Total Carnage" and play through by yourself. The major advantage to this approach is that you're able to save your progress, quit whenever you like, and pick it up later. The downside is that Total Carnage is very challenging and you'll be taking it on alone.2. You can find up to seven other players and play a co-op game on Total Carnage difficulty. The only real downside to this approach is having to coordinate a time with everyone else because you cannot save in co-op games, so you'll need to play the entire thing in one sitting. The upside is that you have unlimited respawns for everyone and you can choose to start a co-op game on Level 18 (The Big House), so you'll only have to play levels 18-28, instead of the entire game. Note that if one player accidently activates a switch manually, it only invalidates the achievement for them, not for everyone in the game.3. You can play a split-screen co-op with a second controller and a spare account. This approach allows you to start your game at Level 18 and have unlimited respawns, but you still cannot save, so you'll have to beat it in a single sitting. This is a great approach if you don't want to coordinate with a group or you have to play offline. Just play through the game with the spare account and leave your main account sitting at the spawn point. That way, if you mess up and activate a switch manually, it won't prevent your main account from getting the achievement.