Ouch. Fair enough CR wise (Hard fight) as long as he was on his own, but the potential for a TPK is strong (particularly if he had time to buff in advance with Stoneskin and Mirror image, and you swapped out Burning hands for shield).
Even with absorb elements, it does like 23 or so damage on a failed save (and none of those PCs are proficient in Dex saves) which is a lot at 6th level for that party. The Cleric and Paladin likely take 45 damage, and are dropped to 0 HP as a result.
Burns a reaction and a 1st level slot.
Question for you: As DM, do you track their resource use during the game (either keeping rough tabs on it, or accurately, by recording it yourself?)
Also, the Paladin and Cleric dont have it, and likely dumped Dex for heavy armor.
Next time have the spellcasters mooks run around provoking AoO. (readying an action to just before the enemy casters turn if necessary) Then hit them with an energy spell from the caster.
Watch their faces when you remind them that they cant cast absorb elements as they've already used their reactions for AoO's.