Hello, I make this topic for documentation purpose, not to say if the skill is worth taking or not.
Please tell me if some things are amiss or can be improved, I'll edit this message accordingly!
Methods
The regular method
Plant X Pine Cones next to each other (X must be a multiple of 2).
Wait for 12 to 18 days for the trees to be fully grown.
Burn X/2 idols next to the grown trees.
Use a Pan Flute to put the treeguards to sleep.
Make the boss attack the Treeguards.
The "aggro tree" method
Plant X Pine Cones next to each other (X must be a multiple of 2).
Plant a single Pine Cone next to the boss to create an "aggro tree".
Wait for 12 to 18 days for the trees to be fully grown ("aggro tree" growth doesn't matter).
Burn X/2 idols next to the grown trees ("aggro tree" must NOT get transformed).
Use a Pan Flute to put the treeguards to sleep close to the "aggro tree".
Make the boss destroy the "aggro tree" to wake up the Treeguards.
Bosses
Ancient Guardian
Use the "aggro tree" method.
2 Treeguards required.
Ancient Guardian dash attack will destroy the "aggro tree".
Ancient Fuelweaver
Use the "aggro tree" method.
6 Treeguards required.
Put the "aggro tree" as close as possible to the Odd Skeleton. Giving it the Shadow Atrium will break the "aggro tree".
Unseen Hands phases must be done manually.
Bearger
Use either method.
4 Treeguards required.
Deerclops
Use regular method.
2 Treeguards required.
Eye of Terror
Use regular method.
4 Treeguards required to kill it in a single night.
Woodie needs to stay far away to not get the boss aggro.
Nightmare Werepig
Use the "aggro tree" method.
4 Treeguards required.
Phase 0 (Parasitic Shadelings) needs to be done manually.
Nightmare Werepig dash attack will destroy the "aggro tree".
Pillars dreadstone might not be destroyed.
Woodie needs to stay far away to not get the boss aggro.
Shadow Pieces
Use the regular method.
8 Treeguards required.
Knight must be killed manually before triggering the Treeguards.
Woodie needs to stay far away to not get the bosses' aggro.
Ink Blights
Use the regular method.
2 Treeguards required.
Toadstool
Use the "aggro tree" method.
8 Treeguards required.
The Treeguards will push the Toadstool over time, they must be placed on the void side to push Toadstool away from the edge.
The "aggro tree" must be planted as close as possible to Toadstool. Chopping Toadstool will destroy the "aggro tree".
The Sporecap phases must be done manually.
Misery Toadstool
Same as Toadstool, except 20 Treeguards are required.
Undoable / Not ideal
Sea bosses
Crab King
Malbatross
Bosses Treeguards can't aggro [reliably]
Antlion
Bee Queen
Celestial Champion
Deadly Brightshade
Dragonfly
Klaus
Lord of the Fruit Flies
Moose/Goose
Spider Queen
More than 20 Treeguards required
Twins of Terror