in singleplayer heroic, you take between 50 and 100k dmg per enemy bullet that hits you from most full auto weapons (assaults, grenadiers, and such not snipers, pistols or shotgunners)
so you even if you have 4 sloth with max regen and 2%/s from kneepads and brandset, plus lets say 1.3m armor, you have 46k regen
this means you need upwards of 1s for each single bullet that hits you, so as long as you can avoid getting hit to often it can work, but since you have to give up a fair amount of dmg to achive this regen its not easy to avoid getting hit at all
in challangeing the dmg you take is ~60% of what heroic does so you can regen most bullets within 1.5s and due to the lower health of enemies it is possible to not be hit to often
but for htis you need supirior awareness and permanently outplay the AI which is to say DONT sit in a single cover and wait for the enemy, be agressive and mobile just not so much to kill yourself, utilise skills like the decoy to distract and then outflank the enemy, aim for the head and weakpoints to improve your dmg, and above all, know when to run and what route to take so to not die on the way to temporary safety where you can use a armor kit
for this to work initiative is the most important part, if you prefer to sit back then this will not work, as the ai will just push you into the corner and overwhelm you unless you push back and bringhte fight to them
Even if you're on full tanker build of 1.7 million armor, that's like 17k regen per second. If you're running TP, the white flag can regen for 1 % per hit, up to 10% per second, which is still considered crappy. in PVE, high difficulties mobs can hit about 100k per hit, a grenade can hit near a million, sniper can one shot. This 1 % is just a waste of slot, even the lowest red roll can do better.