To defeat the Materia Guardian boss in Chapter 1 of Final Fantasy 7 Rebirth, follow these detailed steps:

1. **Understand the Boss Phases**
The Materia Guardian has three distinct phases, each with unique behaviors and attacks:
– **Phase 1**: The boss remains on the ground, attacking frequently with Dual Tail Lash and Seizing Claws.
– **Phase 2**: The boss leaps onto the ceiling, spamming Mucous Jet.
– **Phase 3**: The boss leaps into a pool of mako halfway up to the ceiling, then returns to the ground, becoming more powerful.

2. **Choose Your Active Character**
It is generally recommended to make Sephiroth your actively-controlled character. Sephiroth has access to a wider array of spells and abilities, and his physical attacks are faster.

3. **Positioning**
To avoid taking unnecessary damage, stay behind or to the side of the Materia Guardian. This will help you avoid its foreleg-centric strikes, including Dual Tail Lash, which can inflict significant damage.

4. **Switch Active Characters**
Periodically switch your active character for around ten seconds to avoid getting stuck in front of the boss. Enemies tend to target the player-controlled character more frequently, so you can take a few hits and then swap back to Sephiroth.

5. **Manage Attacks**
– **Phase 1**: Focus on avoiding Dual Tail Lash and Seizing Claws. Use Sephiroth’s abilities to attack from a safe distance.
– **Phase 2**: When the boss leaps onto the ceiling, use Warp to zip up and target its hindleg, which is less likely to be attacked by the boss.
– **Phase 3**: After the boss absorbs mako, its stats will improve, and its speed will increase. Stay to the side and use synergy attacks with Double Helix and Limit Breaks to whittle down its HP.

6. **Synergy Attacks and Limit Breaks**
As the fight progresses, use synergy attacks and limit breaks to take advantage of the boss’s increased speed and power. Keep whittling away at the boss’s HP until it activates its synergy attacks or limit breaks, which will give you an opportunity to deal significant damage.

7. **Trophy/Achievement**
Completing Chapter 1 will also help you earn the trophy/achievement “Never Meet Your Heroes” if you’re aiming for it.

By following these steps, you should be able to effectively defeat the Materia Guardian boss in Chapter 1 of Final Fantasy 7 Rebirth.