>>26462808A lot of the requirements and drawbacks of doing the Synchro with Greninja rely on Ash.
Both Ash and Greninja's heartbeats when linked by the synchro are one, they're pretty much linked together. When it comes to battling, any damage Greninja takes Ash receives as well in the form of pain and this has already been shown that too much pain can cause Ash to de-sync with Greninja and cause him to pass out.
In the episode where he battles Diantha, it showed both the heart rates was extremely fast and rapidly approaching high levels, and that was because Ash was filled with an adrenaline rush. He commanded Greninja to attack even though Mega Gardevoir was down on the floor.
In the following episode, ash ended up forcing the synchro which is why it failed. His hear rate was disrupted and all over the place, making him unable to sync. That kinda shows that Greninja has no problem going into the form, but Ash has to be able to regulate his heartbeat and sync it with Greninja.
So a lot of the problems fall on Ash, but they'll be solved in this week's episode when he masters the form.
