I don't know much about eventing for battles or skills, but this looks good and was easy to follow.
A suggestion to make the event a little (lot) shorter:
Change Skills: [Vince], - [World's End]
Conditional Branch: [Sinu] is in the party
Conditional Branch: [Mae] is in the party
Change Skills: [Vince], + [World's End]
Branch End
Branch End
I'm not sure where the appropriate spot for Erase Event is there (why you do it when you remove the skill but not when you give it?) I'm also not sure why you USE Erase Event and not just Exit Event Processing or turn the switch off, but as I said - battle and skill events are new to me.