Aside from this "acolyte set" (which from a brief glance does not appear to have anything to do with only taking one damage) usually if people are taking low damage that means they have high def - in particular, with 100 def you would take 1 damage from all physical melee attacks. That's the most probable reason for only getting hit with 1 damage, but as noted acolytes have skills which boost their def to undead monsters, and furthermore there are plenty of damage reduction gears as well (tirfing, deviling, and more). Another thing that helps is assumptio, if you're a high priest - or you can use a soul ring if not. There are plenty of ways to lower the damage you take - getting it down to 1 is pretty expensive, but you can at least get it pretty low without spending too much money (depending, of course, on how you define "too much")
