I haven't tried recording it, but if nothing else the "time consuming" part would still be true if you need to resort to that.
You press A on the main menu (where your trainer photo is) at the same time that you start EonTimer. The pre-timer, if I'm not mistaken, is supposed to account for the loading time of the game, so that the actual timer can start itself as soon as the game is fully loaded. After adjusting it on my first few attempts, I ended up with a pre-timer value of 2600, using my Old 3DS XL (different systems will likely be different, N3DS will be closer to 1500). You should notice the game music starting right around the time that the pre-timer finishes. In your waiting time, talk to the statue and wait on the textbox where the Tapu says their cry. Press A at the end of the second timer, which will immediately start the battle.