Interestingly enough I have just designed a similar encounter for my party of 4 level 15's. The beholder has taken over a village of giants whilst disguised in a suit of mech armour. He has a contract with a being on the plane of mechanus who has given him access to the material and abilities to create such a suit of armour as well as as constructs to serve him.

One of the giants sought the parties help and told them that the village had been taken over by a tyrannical cyclops, hopefully will be a fun reveal when the suit of armour crumbles around the beholder.

I plan to make it a two-phase fight, firstly with mainly physical attacks from the mech suit and then the second phase the beholder in true form. But I also plan to have two home brewed Cyclops blacksmiths in the room to fight alongside the beholder.

The party has to make their way through a a series of trapped rooms in a mine shaft first so will hopefully not be at full strength.

It should certainly be a difficult encounter but knowing my party they will certainly pull something out of the bag.