Are mindflayers much nerfed on 5e? Because on BG2 i had problem dealing with then with high level party members. How they plan to have a low level game with such powerful creatures?
In the unmodded original BG2 they can be very easy.
They can dominate you and they can AoE stunn with mind blast and then eat your brain.
Use one character with immunity to mind effects (e.g. Lilarcor equipped) as bait and when they go after him the others attack from far away.
When you can cast mind blank on the party they become completely harmless.