Hi, and welcome!
If you don't/aren't able to come out to your grandma, I'm afraid she's going to keep misgendering you no matter what. She will also continue to call you by your birth name until she's told otherwise (and perhaps for a while after that, too) - and she will do it for the rest of your life unless you come out to her. So you really have two main choices here: find some way to cope with it but let it continue to damage your relationship with your grandma... or bite the bullet and come out to her.
In terms of coping with it, the only sensible thing you can do is to limit the amount of time you spend in her company during her visit, and just let her get on with misgendering you. But every time she does it, remind yourself of who you are, and comfort yourself with the knowledge that this is a temporary situation. And cut the conversation short if you're uncomfortable. She's only visiting; she won't be there for long and then you can get back to normal again.
The thing is, I don't believe this is a very healthy way forward. You see, despite her apparent transphobia, your grandmother does have a relationship of some sorts with you, and it may be important to you to keep that relationship going. If you're planning eventually to transition, she's going to have to find out at some stage anyway. Would you be OK with that happening when your hormones are up the creek and you're trying to adjust to your RLE, or would it be better to do it now? She'll find out eventually anyway.
You hinted in another post that you've come out to your family. Is there someone else in your family who can help you with your grandma? Could your family back you up and tell her on your behalf that you go by a different name and pronouns now? A grandchild on their own is in a very weak position against their grandmother, but if several family members tell her the same thing it might strengthen your case.
At the end of the day, if your grandma pitches a fit when she finds out you're trans*, she was going to pitch that fit anyway. A lot of us have family members who swore up & down that they'd never have anything to do with us ever again... but for many of us, those family members have eventually come round and accepted us. Those that haven't come around are regrettable, but our lives can be happier without their negative influence.
