Ogre Dust has 11 archetypes, which are basically classes. Each archetype has a theme and does a specific sort of set of things. Druids, for example, like to control terrain. Ninjas? They're opportunistic murderers. Elementalists, though, are evasive blasters. In Air form, they can zip around the battlefield. In Fire form, they are ranged damage dealers with a healthy attack. In Water form, they provide cover. In Earth form, they're (relatively) durable and can push people around.
It doesn't take a lot of imagination to see different ways that an elementalist can be useful with just those abilities- if they need to, they can re-position using Air, and then settle down into Fire and deal some damage. If somebody needs space, they can Air into Earth and try to generate some. Water is a useful affinity for traveling while avoiding ranged damage (1), although it's not great (because Elementalists aren't great at affecting the battlefield in quite that way.)
Advanced moves for Elementalists make them better in one affinity or the other. Razor Shards gives the Elementalist a unique niche as durable artillery, and Water Jets lets the Elementalist get places a little more safely. Hurricane Gusts (not shown) lets the Elementalist take risks with spacing, by allowing a whirlwind-attack style action. If I didn't mention it before, there's no reason an Elementalist can't strike with a weapon- they don't get great weapon selections and they're not especially durable, but an Elementalist is no slouch in non-magical combat either.
The only Mastery ability I've written so far is "Ride the Lightning," which allows a charging Elementalist in Air affinity to get +10 to strike and 20M bonus damage, which is a good bit. Of course, the Elementalist will end their action next to an opponent who's probably very upset, so it's a risky maneuver that might need a little work to come out. But that's intentional!
The Elementalist is a risky, glass-cannon class that needs a solid frontline and maybe a little support to truly shine. But in the right opportunity, they're the star of the battle, dealing solid damage and escaping without a scratch.
(1) Water "feels" weak because it just gives a sort of pseudo-evasion, but not a huge amount. That'll need a bit of adjustment at some point.