Forsaken Remastered

It is actually surprisingly possible with this exploit:

1. Play level Fishy on easy (You can die, collect both secrets etc. no worries)
2. Create save when you are closing end of the level (After you have destroyed reactors and door unlocks. For some reason I couldn't load saves where timer was on)
3. Quit to main menu or finish level. It doesn't matter.
4. From main menu start new game and go to mission select like you normally chose missions BUT select difficulty "Total Mayhem"
5. Start some random level like Volcano.
6. Load the save which you made.
7. Finish level Fishy without dying (Enemies hit you a lot. Spam transpulse, use pine mines, clear final rooms with Titan. Rush to the exit)
8. Achievement should unlock

The technical reason why it pops is that loading a level resets level cache like heat meter, some scripts (you get messages to destroy reactors even they are already down and level completion time resets...) So you can basically run the level with easy and load back in with Total Mayhem and the game forgots your previous deaths etc. Enjoy.

Well after playing and finishing this level "fishy" a few times with the difficulty total mayhem and no achievement popping, it definitely seemed "surprisingly impossible". So changed the mac address in the xbox settings, nothing. Thought screw it, try one last thing and if that dosent work I'm done with this. So started a new single player game, total mayhem, then loaded a save that i made at the end of the level previously. And pop, got the cheevo. So the game must reset the difficulty of your saved game to normal instead of leaving it in the total mayhem difficulty. Hope this helps for anyone who was stuck like I was!

Practice in easy mode a couple of time.
Use transpulse as much as possible.
Save often (saving and reloading restart the heat meter !).
You can die, as long as you reload a previous save state.