I think probably the first thing to accept is that there are still many,
many people who hold those opinions. Whether through ignorance, belligerence, whatever the reason may be. And that there's no way to know whether they are actually transphobic or just haven't had access to information and are forming knee-jerk opinions out of hear-say and whatever else.
In that regard, perhaps a suitable thing to do would be to inform this person/people that there is a wealth of information out there if they would like to read up on the subject. And if not, then really there's nothing else you can say, other than you hope they change their narrow-minded view at some point.
Fighting fire with fire leads, more often than not, to an inferno from which everyone gets burned. The most disarming way to approach things is by being kind, calm and considerate. When you don't rise to someone's obvious baiting, they don't know what to do. After all, you know what's right, and who you are. The most you can hope for is that the people you're speaking with will go off and get more information on the subject. If they do, and maybe their views change... all well and good.
If they don't, sadly that's their choice. You can't change their views for them. They have to do it themselves. You just say what you feel you need to say, and leave it at that.
To quote Morpheus:
"I'm trying to free your mind, Neo. But I can only show you the door. You're the one that has to walk through it."That's my view, anyway.