Moonlighter

This achievement requires you to, as it states, kill 1000 forest enemies. This applies to all enemies found in the forest dungeon, although I am unsure whether it applies to the uncommonly found 'pockets' of forest found in the golem dungeon (however the kills from these would be so negligible, it's almost not worth mentioning).

The best way to approach this is to take your time visiting every room possible, killing everything before you descend a floor. The numbers of enemies is random, but I was averaging between 100-120 kills per run to the, now empty, boss room (after killing the boss, the final room has a bunch of regular enemies).

It's best to combine this with the achievement for reaching the 3rd floor of the forest dungeon 10 times; if you don't get this achievement before that one, you will be very close.