Awesome story from you! Not the best but still good in my opinion. I am shocked that not many people reading this fanfic. Althought I did spotted few mistakes like:
'lightning', not 'lighting'
"Well done Luigi, YOUR a master...", change the word ''your'' to ''you're". (I don't know how many times I encounter this same mistake in others' fanfics or the ''you're" to "your" in this site as I read a lot of fanfics at here). Same goes to "Luigi, I'm impressed, even when YOUR afraid."
What the heck is 'fnesed'? I can't find the word in my dictionary, and I don't think it is a word. Do you meant like Luigi froze on the spot, from the pain or he is shaken/shuddered due to the pain or fear?
"Hiding a snarl, IT'S red quartz crown...", are you stating King Boo is the crown? Change the ''it's" to the word "its"
'Boolossus impaled the horse's horn.' What? Wasn't Boolossus was inpaled by the horse's horn?

Also a small advice, you don't need to put brackets as they are not necessarily in here as they are part of the actions that the characters are showing. And some of your brackets had no starting or ending.
Anyway, nice chapter and this should be viewed by others as I read this fanfic more than twice.