Well I did do many hours of dedicated training about a year ago to cap Dark, Enfeebling, Divine, and Elemental magic for WHM and BLM, to level 90. Did that by spamming spells on worms in Abyssea Uleguerand for a while. Maybe took 20 hours total for all of them. When they raised the cap to 95 then later to 99, I just didn't have the patience for it (I tend to look at a lot of things in this game in a frame of mind of "is it fun and if not is it worth the effort?). I capped Elemental Magic for 95 quite by accident when I was nuking mobs to do Darkness staff magian trials. It's still not capped for 99 though, and the other skills are all somewhere slightly above the cap for 90 I guess.
But the part where you suggested spamming spells on IT mobs is troubling. You said you'd need a tank, but that's someone to stand around and hold hate for hours upon hours while I spam spells. I have some very helpful friends who would drop everything to help me some times, but even they have limits.
Still, I might mosey on up to Uleguerand again and try those worms once more. I managed to cap H2H for MNK and Axe for BST up to level 99 in a not terribly long time (maybe 6-8 hours). Though those jobs had the luxury of being able to skill against mobs that could fight back. WHM and BLM are a lot squishier.