Not sure wether it's a Demon War feature (actually I think it is) but - using Flaming Weapon or Acid Dripping Weapon is extremely useful in situations like this. Otherwise, usual stuff like buffing with STR (crushing blow and damage increase) or INT (for critical hits) potions might help. Otherwise, just come back later after you've found a better DPS weapon or levelled up a couple of times. If you have a fire source like a spell or an oil potion, luring the mob to a bunch of barrels or a doorway and then setting fire to stuff is useful to. Kiting the mob over traps can work too, I often do this over fire nova traps (since the player can genearlly outrun them but mobs get stuck in the ring of fire) or cave-in traps which can do some big damage and again are easily avoided by the player. Scout the level and try to find haste obelisks or giant obelisks as both of these will significantly increase your DPS. Evil pools can give you the giant mutation as well.
If all else fails, you'll just have to get a level or two and come back later.