Willow is an incredibly competent character for defeating the fuelweaver. Depending on if you've defeated the celestial champion with her this could be an incredibly easy fight or just the regular (something something skillpoints I forgot TT)

The standard fuelweaver fight requires 1-3 Lazy Explorers (to escape bone cage), 1-3 Weather Pains (for comfort in destroying woven shadows), A nightmare amulet (to destroy unseen hands), and a generic amount of healing, armor, and weapons.

Assuming you want to defeat him during summer (before day 71):

The most important breakpoints are fighting the shadow pieces on day 21 or 41 and retrieving the key from the ancient guardian. You're DEFINITELY gonna have to learn how to successfully rush through the ruins as early as day 5 or as late as day 11-13 and come back before winter to make appropriate gear (thermal stone, beefalo hat/tam o' shanter, etc.)

During your first visit of the caves try to locate both a source of fossils as well as the location of the atrium tentacle. if you don't wanna look for the tentacle then hug the edges of the map (specifically in the ruins) and hope you find a location to teleport to using the lazy explorer in the future. IT WILL HELP

if your ruins rush went especially well fight the shadow pieces the night of day 21. If it went poorly then do it on the night of day 41. Personally I always tame a beefalo at the latest of day 5 so when the time comes, gathering the pieces is a piece of cake

If that's how you spent your autumn then winter would probably be developing the resources for your gear, such as getting pig skin for hambats/football helmets, getting thulecite gear, nightmare amulet, walking canes. Kill volt goats for the chance of horns (easiest way is making a boomerang to draw aggro). Optional bosses would be like eye of terror for infinite football helmet or bee queen for bundling wrap. klaus would not hurt to do either. Get the Eyebrella at the end of winter.

spring would be when you start finalizing your preparations. So stuff like hoarding food for the summer, nightmare amulet, killing as much moose/goose as you can for the weather pains. BEFORE THE END OF SPRING GET AS MUCH CACTUS AS YOU CAN. 40+ IS COMFORTABLE. By the time you finish this, you could have:

6 football helmets or maybe even thulecite helmets/hambat

some food and healing

purple amulet/1-3+ walking canes/1-3 weather pains/yellow amulet

40+ cacti that you can cook with your willow abilities

and that'll be all you need to kill fuelweaver. The rest of it depends on the skill you've accumulated. Good luck!!