D&D General Mind Flayer Encounter

[S.Woodlander]

Here's another question that hopefully you guys can help me out - DM's?

For the main encounter the boss encounter, what level mind flayer (and /or how many) should I have for the encounter?
How many thralls?
It's a 2E campaign and the party is players and levels range from 7th to 10th level (and I have one 16th level dwarven cleric).

 

[S.Woodlander]

One other thing, I I am going to add a beholder as the boss - and the mind flayer as its lieutenant maybe... thoughts, opinions?