There's some sort of important tournament where the mane 6 + Trixie enroll themselves into and Twilight almost kills Trixie but is stopped by Celestia

Maybe this one?
https://www.fimfiction.net/story/3378/the-moonstone-cup

7524781
Nope though that one actually did lead me to the one I was looking for, many thanks for the help

This is the fic if you want to give it a read The Harmony games

7524790
No worries. I'll be sure to check it out!