In a nutshell, let's say that I have Mudkip and May has Treeko. Would there be a way I could see Blaziken without trading?
Simple as that. After playing through it 3 times (not for this answer), I have not encountered a trainer who uses the starter weak to yours.
Hope this helps!
I'm assuming that after a while you WILL find one in the Battle Maison.
Hope I helped :)