LEGACY said:

English isn't my first language, so I'm not 100% about this. But to me "This champion has fully upgraded all of their skills."  sounds very wrong. I would write it as "all of it's skills" but I'm not sure if that's correct.

Can anyone confirm what the correct spelling is?

1) This champion has fully upgraded skills.

2) This champion's skills are fully upgraded.

You can't say "champion" and "it's". Champion is "he" or "she" and using "their" is grammatically correct.

Also issue is about grammar not the spelling.