But in part when she show to Froddo whow powerfull she is it seams like a evil elf but ofcours she is come down
This is just a vision of what she would have become had she succumbed to her desire to possess the one ring. You will see this happen again later on with Samwise. Due to the fact that Galadriel is good, and is so powerful, she passes the test. In the time of the Third Age, Galadriel is the most powerful elf in all of Middle Earth. In a way, she is sort of like a witch, just not an evil one. I think the term sorceress is probably more appropriate.
"I would have the Ring-bearer bring the crown to me, and let Mithrandir set it upon my head, if he will; for he has been the mover of all that has been accomplished, and this is his victory." Elessar