Hey, sweet, a chance to be useful.
Anyhow, Fire Leo, while at first the most daunting of any boss you'll face in the game, has a pretty simple pattern, when exploited, can make this battle a breeze.
First, avoid (jump over) him as he runs around the stage and scatters those boulders about. Before he gets a chance to start spewing his massive fireballs, just mach-speed the granitey *** off of any of those boulders until you're red n' glowin'. With this firey aura, you'll be immune to his fireballs, so get your butt down over to him and hit him to extinguish his fire.
Now, you're going to have to engage him in some hand-to-hand combat. Just dodge his attacks and retaliate with your own (preferrably with Red-Hot One Hundred (Zoom, Slow, Punch)) until, and a little after, his shield breaks. Now his gonna go into a spinning mode a la Bianky Ballerina. Watch the skulls and listen for audible clues as to whether it'll be a high or low attack, as there will be a whole bunch of them and in quick succession.
After you dodge ALL the attacks (with maybe one hit leeway), Leo'll get dizzy and be rather vulnerable to attack. Again, I suggest RHOH. After a few hits, he'll be knocked backwards, jump back into the pool of lava, jump back out and the whole cycle repeats itself. Be warned that he picks up the pace quite drastically as his health bar lowers.
As for storing V-Points, all I can suggest is utilise the Rock-On combos whenever possible. Also, don't be afraid to die, as replaying a level a few times can lead to massive V-Point gains.