I never really understood force ghosts. The line “strike me down and I will be more powerful…” doesn’t really become anything. Force ghosts don’t really do much other than mentor. Yoda is the only one who interacts with the real world by burning the tree. Luke can’t even remove Leia’s Saber from the wall.

I feel the “this is your fight” is a line to give Rey a realisation that she’s the only one with enough power in the real world “the last jedi” to take on Palpatine. I’ve reused this line just as the force ghosts appear later to show this is Rey’s fight but they have her back when it comes to her having the strength to force push the final lightning blow, I’ve also rearranged the ghost scene so Luke sticks around to protect Rey from the blast after Palp dies.