How to modify a monster to be a reasonable challenge for a level 1 party?

I was preparing a one-shot adventure for my friends. It will include a fight with a weakened mind flayer and a giant heart which will be the boss of the adventure. The heart will summon gory minions (could be anything, must be suited to the gory theme).

How do I modify the mindflayer to be a reasonable challenge for a level 1 party and what official monster is the closest to a giant pulsing hearth (mabe gibbering mouther?) might still be workable. The party will include four first level player characters consisting of the folk hero fighter, cleric, wizard and a rouge from starter set character sheets. Thanks 🙂