The second Trial is more of a challenge, a capturing Pokemon challenge to be exact. It can be found on Ten Carat Hill by riding on and using Tauros to smash the large boulder off to the right immediately after you arrive.
The challenge is pretty simple; all you must do is catch at least three Pokemon. This is even easier to do with your incredible Greninja and the ample amount of PokeBalls you are given to start the challenge. You can catch as many Pokemon you want to, but only three are necessary to complete this Trial.
After catching at least three Pokemon, Talk to the man orchestrating the challenge to end the Trial. You will release all the Pokemon you caught back in the wild, and then you will receive a gold Nugget from him. Like Greninja, this pricey Nugget can be sent to your actual copy of Pokemon Sun or Pokemon to via the Pokemon Link.