Ring of Pain

Start a game on normal with Double candle, equip something that gives you heal or armor (Chugging Mask should be fine).

Play the game until you find a poisonous creature and a Roaming Poisonbead. What you need to do is basically, lure the Poisonbead next to the other enemy and let him explode in the same turn you attack the other, in that way, you get poisoned by attacking one enemy and poisoned again by the explosion of the poisonbead in the same turn.

There is probably more ways to do this achievement like using a legendary weapon that allows you to attack two enemies at the same time so If you find another easy way leave it on the comments so I can update the guide accordingly.