Heh, which pokemon learns Sunny Day?
Curious if the OP finally beat Aya. After I beat Fern with a lot of struggle, I ended up waiting and training up my Pokemon and swapping out my Infernape for Arkanine.
She's full of surprises, like the Icy Winds, Aqua Tails and Earths Power. I lost the first two times messing around with tactics but finally pushed to get my Flygon. For a Psi pokemon, I recommend Mr. Mime! Substitute is just really nice for the cheap shots she tries, swapping out her Nidoqueen for her Gengar to get off a fast Night Slash to try and take him out of the picture. I got lucky and Dragon Tailed the Gengar for the Seviper and one-shot it the next turn
Also, if you're not opposed to using items, use the battle items! My Arkanine with an X-Special easily put the pressure on her team once she swaps to Drapion by spamming Heatwave.
Another piece of advice I'd give is looking to see what levels a pokemon learns moves before evolving them. You can save yourself some time and frustration by training a non-evolved pokemon high then de-leveling it so you can use it. Like Flamethrower on my Arkanine would be good, but Heatwave is better and hits all enemies. Discharge is a lot more powerful of an electric attack for Magnezone but if you evolve it fast, you have to wait until later after Aya (lvl 51) or train Magnemite to 42 instead! And Discharge paired with a ground type in your 2nd slot or a pokemon with Lightning Rod is a good way to pressure Aya at the start.
And you don't need lvl 44 pokemon but they make things easier I managed it with a 40-42 team + Flygon (lvl 45) as a ringer but only needed him when she started spamming poke-swaps That's when Flygon said SWAP THIS! and the defeat train started with a forced Dragon Tail swap