I am a new World of Light runner who is starting with doing spiritless runs. I've done a NG+ run before but I'll probably eventually do NG runs so is there any guide for when to use skill spheres?

I did the game spiritsless on Easy at 100%, I would use Ganondorf and his Ult when ennemies where not on stage to spike them a 0%.
Yeah, the ganon's Ult spike with its sword (before doing the dash) if there is no ground below.
So I aimed at getting double final smash and faster recovery in the skill tree.

Triple jump can help a lot to stall while in air to use chrom, Ike and Kirby UpB.
Having the move speed and attack damage boost at 0% is a good thing against bosses.
I think you'll need to take "wind counter" too, some stages are impossible without it.

Create the routing for such a category will take a long time, I'm happy to help any time.