To treat stubborn fungal nail infections, I have found that applying heat via a magnifying glass (on a sunny day) totally cure my toenail infection.
I started by soaking my feet in warm water for 10 minutes so that my nails absorbed as much water as possible. This is so the heat can be transferred to as much of the fungus as possible. I used a cheap 3′ magnifying glass from a $2 shop to focus the beam on the entire surface of the nail. I would start by slowly focusing the beam on a section of nail until I could feel a sting. I would then pull back and let nail cool before I did it again. I would quickly zoom in and out so that as much heat as possible could be applied (I even saw small puffs of steam come off my nail). I repeated this process across the entire nail surface.
After a month I could see healthy nail growing out from the base of the nail, and within 6 months any sign of infection has grown out.
I found that just one treatment was enough to treat my nail, but I cannot see any harm in applying the treatment over multiple weeks.