I was going to nominate Ho-Oh to start the defensive backbone, checking Marshadow and CM Arceus and quasi-absorbing Toxic with Regenerator (all of which threaten this team), but I realise there's a problem with this: Rock Tomb. If your opponent's Marsh has it and predicts correctly, they're immediately put at a huge advantage where they can then tear through the team (unless Ho-Oh isn't the main Marsh check), which is not good at all.
Based on this, I'm supporting Fairyceus: it can counter Marsh better than more or less anything else (is there any reason to run Flyingceus over it?), it can do well against Mega Ray and Ultra Necrozma, which would be important if ttar gets removed, and it can sort of lure pdon with toxic, which could be invaluable.
Based on this, I'm supporting Fairyceus: it can counter Marsh better than more or less anything else (is there any reason to run Flyingceus over it?), it can do well against Mega Ray and Ultra Necrozma, which would be important if ttar gets removed, and it can sort of lure pdon with toxic, which could be invaluable.