Your Talonflame's Item does make some sense actually, you'll be dealing loads of damage and getting back decent HP. If you want to, try a Choice Band set, you can start killing loads of stuff from the start.
Talonflame @ Choice Band
EVs - 4/252/0/0/0/252
Ability - Gale Wings
Adamant/Jolly Nature
-Brave Bird
-Flare Blitz
-U-turn
-Will-O-Wisp[kinda filler]
Choice Scarf is completely pointless in Greninja. It needs more firepower, so give it a Life Orb. EVs should be 0/4/0/252/0/252. Also think about replacing Dark Pulse for HP[Fire].
Drop your Snorlax because you need Special Support on this team(spoiler: I wanna change your Venusaur to a Special Attacker. Consider either Florges or Sylveon for the Job, but I'll give a set for Sylveon-
Sylveon @ Leftovers/Light Clay
Ability - Pixilate
EVs - 252/0/4/0/252/0
Calm/Bold Nature
-Reflect/Protect
-Hyper Voice
-Wish
-Toxic
Give Venusaur the Mega Stone because you already have enough recovery with Synthesis. Drop Toxic and Leech Seed-
Venusaur @ Venusaurite
Ability - Chlorophyll
EVs - 252/0/0/252/4/0
Modest Nature
-Giga Drain
-Hidden Power[Fire]
-Sludge Bomb
-Synthesis
Cloyster is fine, but change Surf for Razor Shell.
Replace Forretress with Ferrothorn. Ferrothorn is much more well rounded in taking Special hits.
Ferrothorn @ Rocky Helmet
Ability - Iron Barbs
EVs - 252/4/252/0/0/0
Relaxed Nature
IVs - 0 Speed
-Leech Seed
-Stealth Rock
-Gyro Ball
-Toxic/Spikes
