This appears to be cumulative over multiple runs
Banefangs are green snakes that are fast and deal 3 poison damage when they hit you, so dealing with them with melee attacks can be troublesome. Look out for items that let you poison creatures, especially targeted spells. There are spell books you can find that let you poison the Banefangs on a cool down without risking getting poisoned yourself.
There is no need to kill the Banefangs, only to apply poison. Unconfirmed, but it may be possible to poison the same creature more than once for progress to the achievement