For this cheat, you need a cursed weapon/suit of armor. Also, 
having Rafael's Stick helps save some cash, but you don't absolutely
need it. Also, if you want the effects to be permanent, your level 
has to be maxed at 31. 

Equip the cursed weapon/armor. Now, either use Rafael's Stick or talk
to a priest to remove the curse. Your strength or armor class (depending
on what you equipped that was cursed) will go down. Keep re-equipping 
and removing the cursed item until your Strength/Armor Class goes to a 
ridiculous #, like 9999 or something like that. Now, re-equip your most
powerful weapon/armor, and you'll either kill enemies in one hit 
guaranteed (Super Strength), or take only 1 damage from any attack from
any enemy, including bosses (Invincibility)