oh Idea giratina is the last one the mc breeds before you meet arceus since you need the other legendaries everytime the mc is constantly getting the feeling of being watched every time he is by a mirror or water that is not rippling (since giratina needs a reflective surface to drag people to their world from the anime)
Arceus is a mythical so technically they are a one attempt non-required pokemon. Also Rayquaza so far has been the only legendary that has had a legendary barrier to entry. Perhaps Arceus could require the main legendarys to attempt but depends mainly on what Chainsaw wants. Also the anime isn't a main source Chainsaw uses often, I asked for a lil scene/dialogue with Rayquaza about Deoxys due to their connection in the anime but didn't end up happening ... I think I might not have tested it the right way. (This comment makes me sound like a nerd glasses emoji type of guy lol) (Edit: I just checked it was there :) if you want to see it you need to get Rayquaza before leaving to get Deoxys )