The simple strategy you can use is lower accuracy (like elefauna pollen move) and then just lower the defenses of the elementum. Just lower the defenses upto around 55 and you can just kill the elementum in one single blow(If its level 30 crit). You can just go with a team of lower level crits and gain some extra experience from the battle.
Moves:
Nature's sting: poison for 3 rounds
Tree truncate: 25 dmg
Nature's cry: 25 dmg
Life: +1000 health.
Photosynth: removes the advantage that fire has over nature.
Well not much strategy needed to beat this elementum. Good luck in beating your first elementum.
