Zamazenta-H is the better option for a defensive Pokemon, but you really shouldn't be using either defensively. Zacian-H's bulk is rather unremarkable, it doesn't wall anything really important in the tier, and has zero defensive moves, while Zamazenta-Crowned has better bulk and Dauntless Shield but its defensive movepool is still ****. Necrozma-Dusk Mane is much, much better, with its good defensive typing, Prism Armor, and incredibly defensive utility options, while Eternatus can lay Toxic Spikes and possesses other nice utility such as Dragon Tail and Mystical Fire. Yveltal can stave off the incredibly dangerous Calyrex-Shadow, and has reliable recovery as well as Defog.
This is how you're supposed to use them:
Zacian-Hero @ Choice Band
Ability: Intrepid Sword
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Play Rough
- Close Combat
- Wild Charge
- Crunch
Zacian-Hero @ Life Orb
Ability: Intrepid Sword
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Play Rough
- Close Combat
- Wild Charge
- Swords Dance
Zamazenta-Hero @ Choice Band
Ability: Dauntless Shield
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Close Combat
- Wild Charge
- Psychic Fangs
- Crunch
And here are the sets for the defensive Pokemon mentioned:
Yveltal @ Heavy-Duty Boots
Ability: Dark Aura
EVs: 252 HP / 4 Def / 252 SpD
Careful / Calm Nature
- Snarl / Knock Off
- Roost
- Defog
- U-turn / Toxic
Necrozma-Dusk Mane @ Leftovers / Rocky Helmet
Ability: Prism Armor
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Sunsteel Strike
- Moonlight / Morning Sun
- Stealth Rock
- Toxic / Knock Off
Eternatus @ Black Sludge
Ability: Pressure
EVs: 240 HP / 252 SpD / 16 Spe
Timid Nature
- Dynamax Cannon / Dragon Tail
- Recover
- Toxic / Toxic Spikes
- Mystical Fire
Lunala @ Heavy-Duty Boots
Ability: Shadow Shield
EVs: 252 HP / 4 Def / 252 Spe
Timid Nature
- Roost
- Will-O-Wisp / Toxic
- Ice Beam
- Defog / Hex
