I don't think round 1 and round 2 is that much of a difference. Merlin has more than just bfr in her arsenal, in fact the bfr skill is just one of her many spells. I have full info on Merlin and minimal knowledge on Aizen, but i'm going to assume that he's hard to kill, catch, or defeat. Is base aizen city or mountain level? Island?
Anyways the thing about merlin is she's hax over hax. Her skills are hax: A spammable, no cooldown, instant cast time, mass teleportation spell, A cube that absorbs and reflects all damage received, A spell that nullifies other spells, An ice sealing spell, An age stopping spell and A tornado that sucks the target enemy to a point that cound fit into a test tube. And the hax on her hax her ability INFINITY, which allows her spells to have infinite duration.
Merlin only needs to land 1 spell on aizen and she gains a permanent advantage. If she lands a tornado then even if the opponent is immortal she'd still win by incap. If she manages to figure out that aizen uses mindtricks to fight then she could just cast a magic to shield her mind and she's safe from mind tricks forever. If she could catch him with a perfect cube then its another gg by incapacitation.
note that i have very little knowledge on aizen hence the "if's". But due to merlins ability, every advantage she gets is permanent. Her ability is so hax, she can freeze the whole world (freeze as many areas as she can, replenish mana, repeat), it's borderline NLF.
I repeat that I have little knowledge on aizen, for all I know maybe he could oneshot merlin right of the bat, but for now merlin both rounds