Not sure if it is a glitch or not, but I was able to get this one with just 3 Tier V skills (Volcanic Rounds, Feed The Flames & Heatwave) and the Class Tree Mastered for the Pyromancer.
So perhaps it might be worthwhile to re-allocate class points to get all the way across instead of trying to grind out a 4th Tier V skill (I mean, unless you're going for Outrider, First Class in which case you'll be grinding out Tier V for all class skills).