Is a physical set viable? Greninja has a lot of options in terms of physical moves.
Not right now. In generation 6 ORAS, Greninja was explicitly banned to Ubers once it got Gunk Shot as coverage for Protean allowing it to turn the tables on the Specially defensive Fairy types, and used to great effect in Gen 7 for the Tapus (along with Dig for a Tectonic rage to cut down Toxapex and the like).However, with the relative dearth of fairies currently, Gunk Shot is no where near as valuable as it once was. If the Tapus Return, then yes Gunk Shot will probably be a mainstay and with how good U-Turn is, there could be arguments for a physical Greninja, though I don't think you could completely divest Greninja of special attacks either -Protean Ice Beam is just way too good.
Ok, scratch that. I have no bloody Idea what I was talking about. Greninja's attack stats are rather balanced and physical sets have rather valuable coverage including Low Kick and Rock Slide on top of the aforementioned Gunk Shot and the amazing utility of U-Turn. You could definitely pull off a physical set. While Special sets do have a bit more of a punch, overlooking Physical sets does a disservice to Greninja's flexibility and viability.
Choice Band/Life Orb
EVs and Nature:
4 HP / 252 Atk / 252 Spe
Physical Greninja makes for a powerful attacker and revenge killer thanks to its guaranteed STAB and wide coverage and near peerless speed. Low Kick covers the many many Fighting weak targets (Garganacl, Kingambit, Chien-Pao, Iron Treads, Orthworm, Roaring Moon, Ting-lu, Tera Steel targets, etc.) running amuck. Rock Slide has great complimentary coverage with Low Kick hitting Flying and Fighting-Resistant targets like Iron Moth, Volcarona, Skeledirge, Dragonite, Pelipper, Ceruledge, Armarouge, etc.) U-Turn, while slightly redundant in coverage with Low Kick, is still a powerful pivoting tool that rewards switching and keeping momentum up. Spikes rounds out the set, further stacking hazards, punishing predicted Electric attacks and adding chip damage that the pivoting Greninja and its teammates can exploit.
Terastalization is a near non-Factor on this set -you want the STAB on the move you pick, which you get with Protean. On the off chance you are running Night Slash or Liquidation, STAB Tera Types may be worth it, but the coverage Greninja normally provides is hard to pass up, and you can save the Tera for Greninja's teammates. Tera Electric may be good for Corviknight, Dondozo and Toxapex.