Well the only fire mage that I ever hung out with for a while trained with explosion. It's definately slower training than poison misting, but it's an option. These are the guys you see using an evil wings all their lives, for the slightly faster explosion cast rate. From what I observed, they seal and slow a group of monsters (creating a freeze-like effect), then explode them to death. It would be doable without slow but they'd get hit more often, since it takes many casts to explode high level monsters. This build is also slower because they need to max seal asap. The guy I partied with was level 99 with maxed seal, explosion, 11 booster, and some in amp so it sounded to me like he only had a few points in mist... I guess all I'm saying is that there's a third build: the explosion build.
I'm not saying slow is completely necessary I'm just saying it would give you an alternative if you're tired of the constant running around of mist training.