Shaymins are typically played either 2-3 but I can see 2 in this deck. If you can make 1 work, great! You maybe could just do the one in order to cram in Mewtwo, but that's pushing it a little. For now, I would do just that, proxying a Mewtwo. Test that until you can get another Shaymin. If you're satisfied with the 1 Shaymin and 1 Mewtwo, I would still get another Shaymin to play in other decks. Now here, I'm not saying Octillery is bad, it just has limited uses. Sure, it's a great budget card that can/will bolster consistency, but it's meant for decks that are slower and need the consistency. Gallade/Octillery is a great example, where Gallade abused Abyssal Hand with it's Premonition Ability. Unfortunately, it doesn't really fit a fast deck in a fast format. I would like to say that if/when Shaymin rotates, Octillery will be the replacement, but it's likely rotation will take PRC-BKT.