At first, Rey didn't get it.
He expected a more overt 'path' to open up before him, so when the entire war was over, he began to wonder if he missed any obvious signs that would lead to his grand goal. However, he soon realized—with a bit of introspection—that he already had the tools he needed all along.
Firstly, the [Time Stop] Skill that he obtained from Feralia, which was now upgraded to [Perfect Time Stop] as a result of his [Perfect Divine Growth], was a key component in the task he was about to undertake.
However, that was only one side of the coin.
The other side was the most recent Skill that he obtained from the Dragon General, [Time Dilation]—or rather, [Perfect Time Dilation].
With these two distinct but somewhat similar Skills under his control, Rey could find a way to get out of the current situation he was trapped in regarding the Dark Elf before him.