Firstly, there is a thread for gym leader help so if you need help with any other leaders post it there.
In terms of how to beat Aya, the first thing I would say is get more pokemon. You've gotten to a point in the game where Blaziken can't just sweep through teams and you realistically are going to need more than 3 pokemon in the game.
In terms of a pokemon for Aya specifically, I would consider gothitelle. You can get a gothitha back in the Beryl mansion and psychic type pokemon are really helpful against Aya. Her Drapion won't be effected by it though so you'll need something to deal with it.
A camperupt wouldn't be a bad idea as it has lots of moves that can hit both opponents and could be very helpful against venusaur. It would be a little less helpful with its ground as some of those moves get reduced by the wasteland field (although not all of them).
But pretty much, get yourself more pokemon and you should have an easier time.