This is a permadeath mode - once you die, you start back from the beginning of the game. You can however use the defibrillator, and you can quit to the main menu if you feel like you're about to die - going back into Iron Man Mode from the main menu will return you to either the closest discovered Trade Town if you were in free roam, or the last checkpoint if you quit out during a mission.
This is not an NG+ mode, so you are better off doing this on Easy difficulty, unless you are going for one of the other difficulty achievements from this Title Update (absolutely not recommended!
) You will pick up the Assault Rifle and Shotgun along the way as part of the main story - between those and the grenades you won't need anything else if you're playing on Easy.
You can pick up the defibrillator in the 'Canyon Cove' Ark in the south eastern part of Broken Tract, which is likely the second ark you will come across. I never needed to use the defibrillator but used Health Infusions many times. You may want to pick up 1-2 Health Infusion Upgrades (1 is found in each Trade Town, and the trader you will encounter on the roads also has one) and pick up the upgrades in Kvasir's upgrade tree which let you hold 6 (or 8 if you upgrade twice) infusions in your inventory.
You need to get Marshall, Loosum and Kvasir to Level 5 in their respective projects to finish the game. Kvasir will likely happen naturally as you move through the map and come across data pads and any other icons with blue labels (such as fallen rangers.) Loosum requires you to complete orange locations, such as Automated Sentries, Gas Stations and Road Chokers. Marshall requires you to complete pink locations, which are mostly bandit dens. Level 5 for each person is 500 points, so you can pick harder targets for Marshall & Loosum to get to 500 with fewer locations that need to be explored.
In the final mission you will be driving a tank through what is basically an auto-scroller, even on Easy you will be shot a lot, and dying in the tank gives you no defibrillator option. If your tank gets to half health, quit out and load the Iron Man Mode back up, where you'll be back in the tank at the previous checkpoint at full health, so you can try again. Targets in this long series of corridors (tanks, enemies, flammable barrels) spawn from fairly far away so you can shoot enemies from a distance to stay safe.
Be careful when in free roam. At one point, driving (or falling by foot) off a cliff is an instant death. As of 2021 it appears that if you do fall off a cliff, you are taken back to a previous checkpoint and you don't die or lose your save.
A fast run through this game mode without unnecessary exploration can be done in 5-6 hours. The achievement will pop after the credits are a brief cut scene with the major characters in the game.
(H/T to Scotty x360a
for some details provided in this solution, and Mack Stardust
for the updated status of the risks driving/falling off cliffs.)