Flat warts are very hard to get rid of, whether a person is immunosuppressed or not. because they tend to spread in the gneral vicinity, and to return no matter how they are treated, I usually advise light, nonscarring approaches, such as light electrodesiccation (burning with an electric needle.) This doesn't produce the big, ugly blisters that freezing does. Treatment must often be repeated several times, at monthly intervals.
Again, it is sometimes more trouble than it's worth to try getting rid of these, even when the immune system is fully functional. Since they're on your legs (and not, say, your face), why not just ignore them? Lots of people do, and they go away when they're good and ready.
I assume, by the way, that the diagnosis is secure. If a dermatologist hasn't seen them, one should do so. Sometimes other benign, nonviral growths can be mistaken for flat warts.
I also have many flat warts. They grow in clusters and recently they are growing more. I do have a stressful lifestyle in college. First, they started on my arms, but now they are one both arms and legs. What can I do? Will they ever go away? I noticed them 5-6 years ago.