Were they always this trivial?
Demonknight: open the mirror, wait one round while the clone rips it several new ones, shoot it with magic arrows. The only "difficulty" in this fight is to not let the clone steal your kill and therefore XP.
Aec'Letec: so slow it can literally be kited around the room until it dies. Only real attack is Death Gaze, fully blockable by Potion of Mirrored Eyes... or Magic Resistance... or just by saving vs. Death(at no penalty) The fight can only be in any way difficult for first time players who will be caught off-guard by the Death Gaze and the demon itself respawning through Cultists.
Oh, and did I mention neither sees through invisibilty so they can just be backstabbed and insta-gibbed with a lucky crit? Or that Aec'Letec's melee attacks don't even count as magical weapons, so that you can simply polymorph into a Mustard Jelly and be completely immune to everything he does?
In a way, Demonknight being easy is fine - he's not really the boss of Durlag's Tower, just an invader, and you use a magical doodad to win the fight based on information form a party that was nearly wiped before you. But Aec'Letec being an easier fight than Cultists trying to summon it is just bizarre.
My recommendation: simply making both being able to see through invisibilty would go a long way. In Demonknight's case, at least you would need to risk getting close to uncover the mirror. In Aec'Letec's case, it would make it more difficult to kill all Cultists before the fight. Increase Aec'Letec's movement speed at least to that of a human. Make his melee attacks count as magical and maybe give him a bit of elemental damage so that Stoneskin doesn't make you completely invincible(Demonknight at least can dispel it) Maybe make it aggro when you attack the Cultists.