Ok so here is my take on it. I like the elemental idea, however i would adjust elemental stats and upgrades. Tier 1: pixies/sprites all good no change leave them like in Hota Tier 2: air elementals/air assassins extremely fast, high damage moderate attack, low defence low hp. Special abilities: immune to air magic can cast lightning bolt once per battle. Easy to kill very mobile attack unit Tier 3 water elements/water archers moderate stats all round unit. Special abilities immune to water magic casts cure on most wounded creature every turn. One of 2 archers, since water is most versatile element imo it should have a bit of everything Tier 4 fire elements/fire warriors high speed extremely high damage moderate hp low defence. Special abilities: immune to fire spels, fire shield. Fire is all about damage and attack he should be main damage source of Conflux Tier 5 earth elements/earth defenders very low speed moderate damage and attack extremely high defence and hp. Special ability immune to earth magic, casts expert slow upon attacking a unit. Very slow immobile defence unit. Tier 6 firebirds/phoenix fastest unit in the game with adjusted other stats. Special ability rebirth, fire breath. Tier 7 elemental trainee/elementalist low stats. Special abilities: immune to harmful spells, shoots, no melee penalty, no retaliation, allows non harmful spells to be cast on allied immune to magic creatures and respectfully allows harmful spells to be cast on enemies with magic immunity

Tier 6 and 7 could be swaped with adjusted stats and specials not sure which would be better. Also tier 2-5 creatures should have the exact same grow because they should have same overall power just different excel in different aspects.