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 )