First post, but I love Trevent, so I had to jump in.
I’ve played about 50 games with this deck on PTCGO, trying quite a few builds and the best for me has been something similar displayed in the vid. The strength in the deck is getting up Trevenant in the first turn so your whole deck needs to work to make this happen.
A 4-3 line for Trevenant with 4 Wally’s gives the most consistent results, but its hard to make room for 4 Wally’s, but I wouldn’t go less than 3. While things like Dusknoir sound good you want to have as few Pokemon as possible to give you a higher chance of grabbing Phantump and Shaymin straight away,
Add in as many Trainer’s Mail and Acro Bikes as you can fit to help you get whatever peice of the puzzle you’re missing on turn one as well as helping to draw through your deck to find exactly what you need throughout the game.
Drop your N’s down to one or none. Wally is going to be taking up alot of supporter space, and you don’t want to get stuck with a hand full of supporters that all do the same thing. Since we have Shaymin ( that will take alot of pressure off your draw supporters), this will free up room for us to run more exotic supporters like Xerosic and Team flare grunt if you so choose and even extra lysandre’s.
You only need 4 DCE energy most of time, so I normally just run with that. Since every turn you’re just reattaching the same energy to Shaymin for the hit run tactic, ou don’t have time to put any energy on Trevenant. By constantly hit and running you’re putting huge pressure on your opponent by drawing all your item and support cards.
I like to run as many cards as possible that remove/slow down energy since it’s really the only thing that can hurt you once you are all setup as Trevenant doesn’t have the biggest body, and only a couple of hits will knock him out.
Obviously Yveltal is the big problem matchup so things like Team flare grunt and Headgear will help us out here, which will also help us out with the Seismitoad match. If you gear the deck for that perfect turn one setup you will more often than not get your item lock in before Seismitoad and if you are running more lysandre’s and/or Team flare grunts you are in better shape to break their item lock. Still really difficult match up though since you lose your muscle band once you’ve attacked. One tip I can give is try to only leave Trevenants on your Bench so you make yourself immune to Lysandre and you’re Item lock can never be broken. However this does leave you open to N.
Looking at it though, I think once people adopt Shaymin into Seismitoad it will be even stronger than it is now. They will not need the Wally’s so it frees up room for things like AZ and Super Scoop Ups for insane drawing potential.
Hope this helps.