A thought on that: Maybe give the enemies an glowing aura that shrouds them, with a pattern and color that resembles the element in question. EG: A martial artist who specializes in Earth Bending has pebbles rotating around their image, or a Pyrolisk with a red haze.
This would also allow you to give elemental phases for enemies or multiple elements that combine patterns. The big downside is that it would require TLC to look right for each creature, and might cost money.
The simpler route is to give enemies a dedicated "game space" for their status info, including elemental strengths or weaknesses. In that case, maybe a tiny picture of the element with an up/down arrow + sword/shield, as appropriate to the creature characteristics.
I guess one of the mechanical questions about the whole thing is how elemental preferences work for characters. Say, an enemy that doles out fire damage, also be vulnerable to the same? I am thinking in this something like having a flamethrower backpack that can go kaboom.