The last time I tried getting the Untouchables I had to use notepad to keep track of the received damage and it felt pretty silly real fast...
I simply didn't take any medkits/globes/zerks/healing levers and only took three invulns far in the game at once that lasted me until the mastermind. It made the game a little tougher (eg. I could only run once per level) but that at least allowed me to stomach the guilt from playing on ITYTD =)
You get 50 HP from the start on a non-marine, so as long as you finish the game without taking any healing, you should get the best untouchable medal.
At first I tried to get the untouchable badges on AoHu and I took notes of damage as well, but then I realized that the method I described is simplier and ITYTD without healing isn't much easier than AoHu, since you don't get the mods in the beginning (almost guarateed tactical shotty) and extra loot from arena (A-modded phaseshift armour? Yes please!), not to mention the mods you can get in the armoury. Since I planned to run through hell anyway, the increased numbers of enemies isn't so big of a deal.