How To Easily Catch Eternatus?

Unlike Zacian/Zamazenta, your capture battle with Eternatus will occur as a Max Raid Battle near the end of the Main Story. During this Max Raid Battle, Eternatus will be in its Eternamax Form, a form that it will only be in for this very battle. Your trainer will team with Hop, Zacian and Zamazenta in order to defeat Eternatus, since your Pokemon's Attacks will be ineffective until the Cover Legends arrive on the scene.

Eternamax Eternatus will remain at Level 60, and it will know the moves Cross Poison, Dragon Pulse, Flamethrower, and Dynamax Cannon. Since it is a Poison/Dragon Type Pokemon, Eternatus is especially weak to Ice Type, Ground Type, Psychic Type, and Dragon Type Attacks. Both Steel Type Pokemon and Fairy Type Pokemon tend to do well against Eternatus, but it does have moves to still hit those types hard. Regardless, the efforts of yourself, Hop, Zacian and Zamazenta should see you easily defeat Eternatus no matter what Pokemon you use in battle.

At the end of the Max Raid Battle against Eternatus, you will have the opportunity to capture it. Believe it or not, you will have a guarantee capture of Eternatus regardless of Poke Ball you use since this is considered a Solo Max Raid Battle because you're teaming with Non-Playable Characters.