> Hmm... I see. A somewhat passionate and stubborn person with a(n usually) good self-control would be a match?

The part about good self-control is a central part of pretty much any elemental magic, at least if one desire's mastery of it rather than simply the ability to use it. While I think Aemoten could potentially have some difficulty achieving compatibility between his personality and the feeling that feeds water magic, I think he's actually taken a good few steps in the right direction by being capable of separating himself as a person from himself as a warrior. Being able to suppress emotion is even more important to an elementalist than to be able to conjure it; most of the magical accidents that happen, happen because fledging elementalists got too riled up about something or another. To a person with natural affinity for magic, cases of strong emotions of the kind that feeds their native element can cause the magic to manifest on its own.

> (What about earth?)

Earth magic feeds off confidence, determination and psychological anguish, and must be controlled with the force of a relentless will. Lightning magic relies on excitement and urgency and is guided by the sincere wishes of its wielder, and the two last of the "primal" elements of magic, light and darkness, are associated with despair and pleasure and hope and agony respectively (no, I did not switch hope and despair), but are both controlled by a very focused, "hands on" application of willpower.

> To the bit about the dangerousness of bombs and their components: Right, that's why I asked about making them, that way Gregory could just brew a couple up before he heads out to battle or before going out on the road. Now that I think about it, if he's a skilled fire elementalist, could he just create a bomb with that? If so, that seems the best path to take. No muss, no fuss. 

"Create a bomb with that"? Eh, as Shien said elemental magic requires constant focus to maintain; using it to infuse something with magic isn't entirely possible in that sense. All kinds of elemental magic are capable of causing explosions in one way or another, though, so if you're going down that road I figure he might just as well use his fire magic to cause an explosion on the spot rather than storing the explosion in something before detonating it.

Edit: Right, it seems you actually meant to ask about the last thing I mentioned. Yes, one could do that.