Based on the first time playing through with this character: interesting mod. I like the whole element mechanic as a secondary energy type, but a lot of things about this character make playing it feel slow, and getting enough elements to play cards is one such thing at the beginning*. It also feels like there's not a lot of meaning to either leveling up (you get to accumulate a lot of emblems or you get to draw from an increasingly large card selection; I'd honestly suggest removing this**, because it's not only adding to the cluttered feeling of this mod, but will also make balancing elements so it's not so much of a slog at the beginning of the game versus at the end much harder) or specializing is almost entirely meaningless (it'd be cool if specializing actually impacted element use or gain in some fashion; perhaps make the the Graduation Letter a character-exclusive boss relic and give it a buff? This would also help bring the Elementalist in line with other characters having only one starting relic).

In addition, so far I have only seen one card that benefits from you having a lot of an element, and that's the rare card Heavensfall, which benefits regardless of element, and lacks an effect outside of its elementcast one. EDIT: Having done a few more runs, I have now seen quite a lot of them, so nevermind.
Also haven't seen any character-specific relics yet, but I'm going to chalk that up to other mods diluting the relic pool and also possibly just not having unlocked any yet.

Some of the "for every x cards in discard pile" cards seem really overpowered, given that you can end up with a 12+ card discard pile only 3 turns into the fight, which, if x=3, means 4 triggers of an effect, which is very difficult to balance given that it can be 0 on the first turn and, with a thick enough deck, 7 on the 5th; not sure what to do about these; maybe adding a baseline effect paired to the for every effect, and either reducing the payoff of the for every effect or increasing the threshold. Either that, or make it depend on your draw pile's size rather than the discard pile; guaranteed not to be useless on the first turn, and can be made useful right before shuffling your deck if you have access to draw.

Could maybe do with a card that gives them energy on the next turn, a la the base characters. Of course, maybe you have added in a card like that and I just haven't seen/unlocked it yet. Sorry if that's the case. Same goes for a card that provides draw, in line with Battle Trance/Acrobatics/Skim. In general, I think more cards not based on a specific element might be a good idea, or even just more non-element based cards in general; this would help with balancing the aforementioned "early-game element slog" issue.

I also wonder if making elements have inherent use effects, which in some cases are modified by cards, rather than elements only having effects as a result of cards, would make the elements feel more meaningful. So water for example baseline having the effect of "for every water used, gain 2 block, apply 1 weak to a random enemy"***, and then having cards like "use two water, apply its effects to all enemies" and "use two water, gain double the block but apply no weakness". That would also give more room for distinct element-specific powers by letting you make powers that add distinct new effects to actual usage itself, rather than just being "first time you use x in a turn" stuff. Or powers could increase the baseline effect (e.g. "water gains 1 more block per usage); this is the sort of thing the aforementioned possible buff to the Graduation Letter and specialization could involve. Of course, this would require a lot of fiddling with the exact usage amounts per card and baseline effect amounts; I think the ideal balancing point might be one where boosting the main effect of each element by one is the sort of thing a rare relic could do without being entirely OP.

In summary, needs some snappier more instant-payoff cards and some of the things this character can do need to feel a bit more meaningful. Also their blocking capability really needs to be nerfed; being able to generate 300+ block in a single turn with the hodgepodge deck I ended up with on my first run seems wrong.

Of course, could just turn out that all of this is a bunch of inaccurate observations based on a fluke of a first run; if that turns out to be the case, I'll probably post my revised observations in this discussion. No matter what, I'm interested in seeing how you end up addressing stuff like the obvious "I don't have good access to this element, so all cards with that element are auto-reject****" vs. "this card can use multiple elements, that's a lot to analyze" issue.

*perhaps making their starting relic also just give them 1 of each element at the start of combat? Helps their early burst potential along, giving them enough immediate damage potential that you'll be able to nerf their insane blocking capabilities without making playing the character impossible. Similarly, doing this would mean you'll have an easier time nerfing the stuff that means resource management ends up basically nullified lategame.
**or making "you can add an emblem of your choice to your deck at rest sites" an effect you can get from a relic; uncommon seems about the right rarity.
***fire could then be "deal 2 damage (that can be boosted by vulnerable) to the targeted enemy and apply 1 vulnerable to a random enemy". Not really sure what Earth could be for its main effects, although "add beneficial status to your draw pile" seems like it'd be a decent secondary effect (which is to say, one balanceable as such). Maybe "gain 1 dex until the end of your turn and draw 1 card" could be the Wind effect? Then that card that uses Wind to give you dex would just say "dex from this card is not lost at end of turn" at the Wind part of the card.
****"and any cards randomly generated by stuff like Nilry's Codex or potions have a good chance of being useless"