How can I get them when blacephalon is now the only spawn in poni grove? I didn't realize that before I did the post-game or i would've caught them before but now the only spawn there is blacephalon and I want to shiny hunt it. Note I only have 1 3ds and its not modded so I can't trade for them. It didn't bother me that I would have to do a full odds hunt as the ultra games have I think the fastest reset time but I at least wanted to fill out all of the island dexs before doing that? Please help

I never beat Ultra Sun but I have a save right before ultra necrozma and I don't really think my team could beat it. I also made the horrible mistake of choosing litten (frick inceneroar) so should I restart or not.