To beat the Mindflayer in the combat simulation of Final Fantasy VII Rebirth, follow these detailed steps:

### Step 1: Equip the Right Materia
– Equip First Strike Materia on a character to ensure they can attack first and fill their ATB bar quickly.
– Pair Magnify and Binding Materia with First Strike to allow for Sleep spells to be cast effectively. This will help keep the Varghidpolis out of the battle, allowing you to focus on the Mindflayer.

### Step 2: Start the Fight
– Begin by having your character with First Strike Materia attack first to fill their ATB bar. This will allow you to cast Sleep on all three mobs (Mindflayer and the Varghidpolis) immediately.

### Step 3: Cast Sleep
– Use the Sleep spell to incapacitate the Varghidpolis. This is crucial as it prevents them from interfering with your attack on the Mindflayer. If any of the Varghidpolis break the Sleep with an auto-attack, reset and start again.

### Step 4: Build Synergy Ability Energy
– Focus on building three synergy bars on both Cloud and Tifa. This will allow you to use a synergy ability that pressures the Mindflayer.

### Step 5: Pressure the Mindflayer
– Use debuff spells like Debrave, Deprotect, and Slow to weaken the Mindflayer. These spells will make it easier to build up synergy charges and pressure the boss.
– Once you have three synergy bars, activate a synergy ability. This will immediately pressure the Mindflayer, making it easier to stagger and deal damage.

### Step 6: Stagger and Deal Damage
– Use Focus Thrust to stagger the Mindflayer as soon as it’s available. This will give you an opportunity to deal significant damage.
– Switch to Tifa and spam triangle to activate Rise & Fall and Omni Strike. These abilities will buff the damage dealt and help you finish off the Mindflayer quickly.
– Once the Mindflayer is staggered, use any damaging ability to finish it off. The key is to keep spamming triangle on Tifa to maintain the pressure and damage output.

### Step 7: Manage Varghidpolis
– The Varghidpolis may use Deafening Screech followed by Self-Immolation, which can kill two of your party members instantly. However, as long as you have all three party members alive when the Mindflayer dies, you are safe.

### Additional Tips
– Be aware of the Astral Apprehension attack and dodge it by looking out for the spots that briefly appear before the lightning bolts impact.
– If available, use summons like Ifrit to increase attack damage and help in the final stages of the battle.

By following these steps, you should be able to effectively beat the Mindflayer in the combat simulation of Final Fantasy VII Rebirth.