So there are a few idea's for a mill deck if that's what you are wanting to run.
Bunnelby (Primal Clash 121) is a great card. Bunnelby has an Ancient Trait called Barrage which enables it to attack twice in a single turn, meaning he can mill two cards a turn for one energy.
Ninjask (Roaring Skies 10) has a neat concept, by discarding one of your cards, you can mill your opponent for one card, this combined with Giant Plant Forest could be a very interesting concept and one I wanted to try in expanded.
Xatu (Roaring Skies 29) as you mentioned has an attack that can disrupt your opponent by making them discard a trainer card which can be great in negating draw support.
Crawdaunt (Primal Clash 92) is also another good idea for disruption. By simply evolving Corphish, you can discard an energy from your opponent's Pokémon.
There are other Pokémon too I believe that also do this, but I can't remember them on the top of my head.
As for Supporters:
Trick Shovel, Red Card (I think it's still in the rotation), Team Flare Grunt, and Xerostic are all good disruption cards that can help you out quite a bit in building a mill/donk deck.
Mill/Donk decks I feel are going to have a rough time in the current format, it's going to have to play faster than ever in order to keep up and stand a chance. If it doesn't, well it's going to get ugly fast (I'm look at you Shaymin-EX.)
Anyways, I hope this gives you an idea on what there is out there.
