Ring of Pain

Soul Spill is a spell that randomly appears in the dungeon pretty early on, as it's a grey item. It allows you to use your accumulated souls to fire off a 1-1 ratio bombardment against all the enemies on the current floor.

Pair this with a Hermit's Tiara Run and max out speed over all other stats. First striking allows freeze on hit, so as long as you keep your speed increasing you can whittle down almost any enemy. Keep in mind you can only use the Tiara if you have 4 or less items equipped and the candle and Soul Spill count against you. I recommend the Ice Breaker ideally as your fourth, or one of the axes that increase speed a decent amount. Swapping Items is allowed as long as no others are added so feel free to take the first helmet and weapon you can until you find what you need.

Farm up to at least 100 souls while having this spell then use it to clear some enemies and the achievement will follow.

Also just to save people time Soul Spill will not kill the Owl!!!!!!. DO NOT SAVE IT HOPING FOR AN EASY BOSS KILL!. I had 960 souls and it did exactly 1 damage to the Owl. Wasted so much time.