Greninja you say? Lucky for you, Greninja is one of the best decks in both Standard and Expanded!
Greninja BREAK is a strategy that involves sniping your opponents Pokemon for a good amount of damage (for snipping standards) while hitting for decent amounts of damage for a low cost. The way it's played means that it can get a good and devastating setup if allowed. It's most major and common threat in Standard is Garbador, but even then it's not a
complete auto loss so Greninja still has a fighting chance. Cards that are most commonly paired up with it are:
Talonflame: Like many other non-basic/stage 1 decks, this one requires quite a few resources to keep a decent amount of Greninjas in play with decent health and able to attack. Due to this, Talonflame, if Gale Wings activates, is a great partner, being able to keep resources coming and for a low energy cost while even doing a bit of damage.
Octillery: Due to Talonflame, it's not so common any more, but the costant flow of resources is always welcome in most decks. Not to mention its water typing allows benefits from Rough Seas and Dive Ball.
Jirachi: This card discards a Special Energy attached to the opponent's Active and gets one-turn invincibility. This can help with slowing down your opponent/rendering them unable to attack while you prepare an onslaught of frogs.
However, Talonflame are expensive and Octillery's crept up in price since it's been released, so for budget options, your best bet would be to either buy a couple Battle Arena Decks (Keldeo Rayquaza) Since those come with great staples and Octillery, or sticking to the Jirachi build.
Some good trainers that are often paired up with it are Bursting Balloon, Rough Seas, Dive Ball, Faded Town and maybe the odd Splash Energy. Then there's the usual staple trainers like N, Sycamore, VS Seekers, Lysandre. They can be a little expensive but Battle Arena Decks come with most of them for a good price.
Since it's a popular deck, there's most likely a few examples of it in the Deck Garage. You can also post your own decks there to receive feedback from other members.
Hope this helps!