The Bible is the best answer to transgenderism
You cannot convince people who have rejected science with scientific arguments.
The book of Genesis tells is that God created man in His own image, and that He created us male and female. From the moment of fertilization, we are marked male or female, and that marking is directly from God and bears His image. When a woman cuts off her breasts or when a man cuts off his penis and testicles, or when either sex ingests harmful chemicals to radically alter the natural chemistry of their bodies, they are defacing the image of God. Transgenderism is a rebellion against creation itself.
Obviously, this does not preclude scientific arguments pointing out that a man cannot actually physically become a woman. We do not live in a comic book, and Raven Darkholme does not actually exist. A man who has himself surgically mutilated is still genetically male, and A woman who alters her body chemistry to grow a beard and chest hair is still genetically female. Biological men who compete against women in sports do have significant (and unfair) advantages such as increased bone density and muscle mass. We should continue to make those arguments, but that is not the only argument we should use.
Some would argue that when we bring Scripture into this argument, we are going to be dismissed as "Bible Thumpers" and that the best path to opposing transgenderism is through scientific arguments. But the transgender cult has already disregarded scientific evidence, and seeks to silence those who present an opposing view. This is not a battle over science and facts, and you cannot convince people who have rejected science with scientific arguments. This is a spiritual and theological battle by those who are in rebellion against the very image of God, and must be fought on those terms.