The fight likely happens in phases.
Phase 1: Homelander shoots heat vision, and Buffy dodges and blocks it. Buffy actively approaches the whole time, trying to get close.
Phase 2: Homelander sees that his heat vision is being blocked and avoided, and sees her getting closer with a sharp blade which he is watching block his heat vision. So he gets angry and flies straight at her. Buffy's scythe has been grabbed by the handle and even taken from her that way before, and Homelander definitely has the lifting strength to do it. All the same, Homelander likely gets hurt in the process of taking it. He might even take a fatal wound right there. Even if he tries to grab her instead of the scythe, she will block with the scythe likely leading to it being taken and her being free anyway. In fact, there's a decent chance that Homelander suffers a terrible or even fatal wound before he can take the scythe, remembering that comic Homelander allowed Queen Maeve to strike him on the head with her sword.
Phase 3: Homelander attempts to bear down on Buffy with her own scythe after taking it, but Buffy's superior skill allows her to easily avoid the swing and evade the heat vision, manoeuvring herself inside the reach of these attacks and landing multiple hits.
Phase 4: At this range, Buffy is relentlessly raining blows on Homelander, who can't even lay a hand on her in response. He will likely attempt to move away using his flight, but Buffy will either hold on and keep attacking or let him move away, knowing she can still close the distance.
The battle might alternate through these phases repeatedly afterwards, if Homelander fights cautiously, but ultimately Buffy is able to avoid all his worst attacks at long range and meanwhile close range allows her enormous fighting skill to dominate. She can also throw weapons at him which will make the long range game even less viable for him.
Homelander can win, and he even has better means of controlling the distance between them, but Buffy has a pretty reliable answer to his advantage at long range, whereas he becomes a sitting duck at close range.