The Groundskeeper is the first boss, in Night Garden, on the way to the Hunter's Cabin. You fight him with the Matchstick and heal with the Oil Vial you unlock just before the fight.
How to beat him
Save at the tent before the arena
Rest at the second Night Garden tent. The path right from it leads to the fight.
Slide through rakes and the leaf stomp
He sends garden rakes across the screen and stomps across the arena surrounded by leaves. Both are too tall to jump cleanly, so slide through them. Your slide makes you briefly invincible.
Dodge the seeds or flowers
He launches seeds or flowers that drift toward you. Move away from them and keep attacking.
Stay close in phase two
At half health his attacks double up. Stay near him, slide through what he throws and hit him in every gap.
He also slams the ground and sends out a shockwave. The two guides disagree on how to avoid it: one jumps over it, the other slides through. Whichever you use, don't start it too early, because the wave is long.
Beating him unlocks the Dark Garden achievement. The Hunter's Cabin is to the right.
Sources: Into Indie Games Part 1, Gamer Social Club 100% walkthrough, Whisper of the House walkthrough.