In this Bulbapedia article, a trailer has been revealed showing of Gen 6 legendary Zygarde’s alternate forms in Sun and Moon. These forms so far are the 10% Forme and the Complete Forme.
The 10% Forme has been shown using Thousand Waves and Thousand Arrows, gen 6 moves that are currently unobtainable but whose coding exists in the gen 6 games.
When 10% Forme is at low HP, it turns into Complete Forme using its ability, Swarm Change, which also restores its HP.
Complete Forme has been shown using a new move called Core Enforcer. This move uses a long animation that ends in a Z-shaped beam bursting at the opponent. Though its type has not been revealed, it dealt super-effective damage to Garchomp.
Seeing as Garchomp is Ground/Dragon, its primary weaknesses are Ice, Dragon, and Fairy. I’m leaning more toward Dragon with Core Enforcer but we won’t know for sure until further information is available.