What is the easiest way to catch cresselia omega ruby

As you have mentioned, Cresselia appears on a rare Mirage Spot called Crescent Isle.

Unfortunately, you don't have any control over which Mirage Spot you get. So, as mentioned by pinckerman in his comment, the best way to find it is to go online through the PSS.

As stated on Serebii.net, you will get the mirage islands of the players you encounter through StreetPass/SpotPass if they have visited their daily Mirage Spot so do visit yours before connecting so you can share it as well.

Once you get to the island, you can save your game and it will still be available if you decide to try and capture Cresselia another day or if you want to soft reset for it, which often takes a few days.