To beat the Terror of the Deep in Final Fantasy 7 Rebirth, follow these detailed steps:

1. **Understand the Enemy’s Weaknesses and Immunities**
– **Weaknesses:** The Terror of the Deep is weak to lightning-based attacks, particularly Thunder and Thundara.
– **Immunities:** It is immune to proportional damage, berserk, slow, silence, sleep, stone, and morphable items.

2. **Choose Your Party Composition**
– **Recommended Party:** Cloud, Tifa, and Barret are suggested for this fight. Barret’s ranged attacks and Cloud’s ability to perform ranged attacks using a dodge are particularly useful.
– **Additional Tips:**
– If you have Aerith in your party, she can attack magically, especially with Thunder.
– Consider giving a Lightning orb to Barret to increase your damage output.

3. **Focus on Ranged Attacks**
– **Ranged Damage:** Use characters like Barret to deal damage from a safe distance. This helps avoid the close-range attacks that can be difficult to dodge.
– **Thunder Spells:** Prioritize using Thunder spells to deal high damage. Thundara is especially effective if you have it.

4. **Manage the Water Cell Attack**
– **Water Cell:** The Terror of the Deep can trap your party in a watery bubble. To break the bubble, use magic attacks against it. If you’re quick, you can evade this attack by dodging at the right moment.
– **Switch Characters:** If a party member is trapped, switch to another character and attack/destroy the orb to free them.

5. **Evade Special Attacks**
– **Grasping Tail:** The Terror of the Deep can grab a party member with its tail. To avoid this, focus attacks on the tail to cripple it, preventing the attack from being used.
– **Oceanic Tornado:** At around 75% health, the Terror of the Deep will disappear beneath the platforms and begin the Oceanic Tornado attack. Focus on avoiding the central tornado and smaller spouts that erupt around the arena.
– **Hydro Shot and Water Jet:** These are range attacks that can be dodged or blocked. Keep moving and avoid these attacks to minimize damage.

6. **Use Synergy Abilities**
– **Synergy Skills:** Use skills like Iron Defense or Bodyguard to block or mitigate damage from attacks like Tail Lash and Spiral Surge.
– **Barrett’s Bonus Round:** If Barrett has his Bonus Round ability, it can power up your shots, helping to get rid of water cells more efficiently.

7. **Stay Mobile**
– **Avoid Bubbles:** Keep an eye out for water rings that signal an upcoming Aquatic Twister attack. Avoid areas with water rings to minimize damage.
– **Stay Out of Harm’s Reach:** Ensure the character you are controlling is out of harm’s reach from bubbles and the Water Jet attack.

8. **Fill the Stagger Bar**
– **Pressure the Boss:** Inflicting damage on the Terror of the Deep’s head while it is in the water will pressure it and cause it to take to the air. This can help you deal more damage and fill the Stagger bar faster.

9. **Use Summons (Optional)**
– **Ramuh:** If you have save data from Final Fantasy 7 Remake Integrade, you can summon Ramuh, a lightning-elemental summoned entity that can deal significant damage.

By following these steps and using the right strategies, you should be able to defeat the Terror of the Deep in Final Fantasy 7 Rebirth.