Vengeful Spirit

Vengeful Spirit (VS) allows you to shoot fireballs in order to quickly kill enemies or perform certain skips in Greenpath.

Greenpath skips
These skips make use of the knock-back when shooting a fireball. Jump as late as possible, tap right, cast VS but do not spam, hold left. Press QUICK CAST rather than FOCUS/CAST. For controller, fireb0rn recommends to swap QUICK CAST and DREAM NAIL so that you can cast VS with your thumb.

If you fail the Fireball Skip in the room where you see Hornet the first time, you can pogo off a Squit to back it up.

Double hits
By shooting in the direction an enemy is moving, you can hit it twice with one fireball.