Firstly, for tips on individual levels, check out Perf3ct Chaoz guide for the achievement
The Legends of Sera achievement in Gears of War 4 worth 505 pointsComplete all Campaign Acts on Ironman in Co-op on Hardcore or higher difficulty
The following method works for most games and will allow you to avoid failing on tricky achievements where a flawless / deathless run is required.
Start your campaign on at least Hardcode mode with ironman turned on. Play through the game in earnest trying not to die. If you do go down/think you're going to die, or die completely press the xbox guide button
. You'll be sent to the dashboard but the game will still be running in the background.
Goto the game tile and press start, then goto manage game. It is important that you do not quit. Selecting the quit option will sync your local save to the cloud and ruin what you are trying to do.
From the manage game options, go down to Saved Data in the left hand menu. You will most likely see a profile save game and a reserved space. Highlight the profile saved game and press
. You'll get 3 more options. Select Delete from Console. This will remove your local save only.
Don't worry this WON'T interfere with your online progress / xp / classes cards or anything like that. This is purely a local copy of a file that will be synced from the cloud server.
Once you've deleted the file it will automatically quit the game so just start as normal. It will sync the save from the cloud back down to your console.
I would recommend stopping play about every 20 minutes/when you reach a new checkpoint you're comfortable and quitting the game to ensure the save file is synced. That way you won't have to repeat too much of the campaign.
To anyone who is sceptical about this working I would recommend you start a new game and play for a few minutes to get to a new checkpoint, quit the game and allow the save game to sync to the cloud. Start the game up again and continue the campaign, then deliberately die. Use the method described to pull down the cloud save you made earlier so you may continue.