How to Unlock the Dark Forest Achievement
To get the Dark Forest achievement you must kill the Lost Forest Lord. Although it is stated otherwise you will find that headshots against this boss will do damage. Mixing a combo of the Light Sword (and blast) + Machine Gun headshots is also effective against this boss. If you use these attacks while continuously dodging around inbetween sword uses and reloading in a circlular motion around the arena you will eventually take him down. The Forest Lord does spawn in regular enemies but they should not pose too much of a problem and the fight in general is pretty easy on the first two playthroughs.Nov 25th 2020, ID#741| REPORT
On the third playthrough the Forest Lord will deal a lot more damage and bullets will not be as effective. You should at this point have every skill unlocked. If you now continuously spam Release Limit (LS + RS) whenever it is available and then Throw Down Lightning (left on D-pad 3 times) and Light Forcefield (right on D-pad) you will eventually take down the boss. While you are waiting for Release Limit to cool down you will need to spam the Light Sword and Shotgun as you run around dodging and Throw Down Lightning.