>>32523558It keeps happening due to game freak's incompetence. They are pretty poor devs, especially when it comes to balancing, but story-wise regarding lore they do a pretty good job, though it's a shame that it rarely is properly represented in-game.
There's no way Complete Zygarde will be able to specifically handle Xerneas, unless it had Aura Break and even then it would be difficult for it to stop it.
And just how they wasted the Complete formes design to be only in-battle is pretty sad, considering it looks so radically different from its other formes and other megas along with primals thooo