A character in my group has multiple ways to reduce/prevent incoming damage, among others the Heavy Armor Master feat and such things. This wasn't a big problem so far. I could just throw more bad guys at this character. And the fact that this player deliberately stacked multiple effects to reduce/negate damage just means, that he wants to play an awesome character, who can take a lot of damage. And that's not inherently a bad thing. But recently this character acquired the ability to cast Armor of Agathys. And NOW it's a problem. Up until now, most combat encounters had some named bad guys and some mooks. This made sense (and was easy to balance), because why would the Big Bad come alone. The mooks helped mainly in the way that the party couldn't allow themselves to only focus on the Big Bad. But since the character in question uses Armor of Agathys, the number of mooks per encounter became practically irrelevant. The mooks just attack, their damage is negated most of the time, and they die a horrible death. This really unbalances my game.
I guess I interpret the spell correct: as long as the caster has the temporary hit points from Armor of Agathys, the attacker takes damage, even if the attacker didn't actually deal any damage. (If this interpretation is wrong, please correct me).
My actual question is: how can I restore my previously held encounter balance, without just taking away Armor of Agathys or all the ways to prevent/reduce damage?