Make sure you have the Music Box and a Friend Bow in your inventory and your lead has X-Ray Specs on. From floors 36-98 in the Buried Relic, track every red dot initially seen on the map. Don't look at the ones that randomly spawn; Mew does NOT random spawn. Your best luck is to find him in a Monster House event. When you finally find a Mew (Three trips through entire dungeon, no sightings yet -_-) be sure that you defeat him while he is adjacent to you. *Throwing Sticks or using a sniper attack will kill him instantly* so don't so that!!! Unless he's next to you that is.
And my Kyogre is purple, too.