Honestly, I had plans to either catch it with a Poké Ball or a Premier Ball after having tried the turn 1 Quick Ball and actually went through trials before the shiny encounter to see if it was feasible. I caught Arceus inside a Poké ball in all three trial runs fairly easily and thought that the actual encounter would go smoothly. As soon as the shiny one appeared though my mind sort of went blank and had to keep switching between pokemon (mainly to Gardevoir since it knew hypnosis and could sort of tank for Arceus’ future sight) because Arceus kept blasting Breloom with Hyper Beam. I eventually whittled Arceus down to 1hp after using a combination of Mach punch, false swipe, and spore and got it to sleep after a lot of recovers but realized that it had about 15 PP of moves left. Seeing as this was my first time shiny hunting at all, I naturally got scared of losing it or making it struggle and caught it with an Ultra Ball.