I love how you did this to the music of Nuclear Fusion.
If only you were calling down a miniature sun instead. Really, that's what the advanced fire magic should be. Maybe even make it so that the spell spawns the miniature sun instead of just calling it down. It takes 10 seconds to do this but it will continue drawing MP from you even afterward (unless you cast the spell), which will further increase the power of the move and add more effects to it. After a while the fusion reaction becomes uncontainable. If you've powered it up enough, mobs will start to gravitate toward the miniature sun and when the spell goes off with one big nuke explosion, lots of smaller explosions will be generated randomly around the area immediately afterward.
You can only use this once every 30 minutes because it takes that long for your milletian immortality to shrug off the massive amounts of radiation poisoning. Attempting to use it before 30 minutes have passed will create a lower-powered nuclear reaction and will also render you unconscious immediately afterward.
I like my imagination. Anyway, this was a fun video.