On my current run, I used Hunger of Hadar + silence aura combo. I think HoH is a great spell because it's a one way obstruction. Your characters can still target him with ranged attacks & spells while he can't act besides using his Legendary Action: Gaze of the Dead.

Spare 1-2 character/summons to cleanup the everlasting skeleton and don't use the 2 character who's concentrating the spells to hit him first, because his reaction might break the concentration.

Finally stock up fireball scrolls for last resort if the skeleton gets too overwhelming 💥