338% is pretty much impossible. I wouldn't try it unless you're extremely lucky when it comes to treasure, or don't mind skipping bosses. Still, if you DO try it, good luck. maybe you'll figure out something crazy that makes the impossible merely insanely difficult.
I have to ask... has anyone actually managed to beat the "Nightmare" difficulty? Even when I'm lucky enough to have all the essentials nearby - housing, food to scavenge, farms, morale structures (bars/churches) and a police station or other defence booster, I simply get swamped too quickly and watch helplessly as my folks have their guts ripped out.
Had a few tries at Harder too, but the same thing happens - it just takes a few more turns. Anyone got any suggestions for surviving Harder/Nightmare for 50+ days? (Already clearing/securing areas with lots of food - not much point in scavenging when securing can get you the farm AND the food, so I usually send SCVs to non-farm areas with plenty of food with a soldier while the other two clear out the zombies from the farms in preparation for a lockdown)...
Oh and I like MadAddams' suggestion about a morale boost for defeating zombies. Good luck to anyone else who tries for “Lasting Legacy” – it is a nightmare...