Zygarde is shiny locked and there is no legal way to obtain it in its shiny form. Soft resetting will be a waste of time. Sorry!
Hope I helped. :)
No, the other legendaries, UBs and Tapus are shiny locked. (except for Tapu Koko, as there will be an event in the future)
A full list of shiny-locked Pokémon can be found here.
Note that not all legendary Pokémon can be obtained shiny when Soft Resetting.