Wendy is going to be over 2.5x faster than AFO at the start, which is a significant speed gap. Her Enchantments are only stated to last for a short time, but I don't see any timeframe mentioned on her profile. I'll admit I'm not sure how to take that.
While Wendy's going to be scaling far above that
35.48 Petaton value, she's still going to be inferior to Rewind All For One. Who scales above
63.59 Petatons, and will continuously grow stronger as he gets younger, strong enough to
one shot characters who scale above
63.59 Petatons for example.
Wendy lacks the stats to take AFO down with her attacks. Even after being reduce to a preteen state, AFO lasted for over 20 minutes, while taking damage that accelerates his Rewind. Since her speed is noticeably higher, that means AFO will struggle to actually land any hits on her, but speed boost is only temporary.
Considering it says for a short time, I think it's safe to say her boost will run out before AFO's time limit does. In that case, he can easily one shot her with a single Impure Beam or any variety of attacks. However, there is one point here that could change the fight in Wendy's favor before that happens.
Wendy has some useful hax, but they're out of character for her to use as it'd kill her opponent.
However, Irene can force Wendy to use it, and I imagine she wouldn't let Wendy die. If Irene uses the Separation Enchantment to separate AFO's consciousness from his body, that's just an instant game over for him. Rewind isn't going to protect him from that. But I admit my knowledge on Fairy Tail is limited.
Is what I'm saying even possible here?
It's also possible I'm missing something obvious from Wendy because I'm dumb.