Caught a Timid HP Ice Nihilego on my first try. It's 4IV and in all the right stats too. However, it seems like my luck is running out trying to replicate the same thing with Xurkitree. This thing keeps killing me and my Synchronizer keeps failing whenever I actually catch it. +2 Electric Terrain boosted Discharge ain't no joke.
It is definitely harder to pull off if you're trying to control an Ultra Beast's Nature with a Synchronize Pokémon, but that Pokémon does not need to stay conscious through the whole battle. Once the battle has begun, and the wild Pokémon has the Nature you want, its job is done. But yeah, due to Beast Boost, the Synchronize Pokémon will probably get knocked out on its first or second turn and raise the Ultra Beast's highest stat.
I heard from others that Xurkitree is the hardest Ultra Beast to catch, especially if you don't have a Ground-type with you. I had a Ground-type (Alolan Dugtrio) and I still struggled big time with both of them.
In Gen 7 the Mega Evolutions get their speed right away.
I got some answers on the help topic as well, but thanks for the reply. This was something I didn't know about prior to that battle...In any case, I'm done with the Battle Tree for a while. At least I got the BP I wanted to get the items I was looking for.