I have a Mew on my team
It only took two attempts to get it to join me
I also have a few tips to get Mew:
Okay, to get Mew, you obviously need to survive in Buried Relic, bring some Huge Apples for your hunt so you won't die before you get it. The X-ray specs are also very good, please note that once you enter a floor, either Mew will appear, or Mew won't appear, if Mew hasn't appeared in the floor, then it won't appear anytime later. So only look at red dots which were there as soon as you entered. Once you find it, immediately switch to your friend bow, and then set your team members' tactics to "Get Away" so they won't find it. If you see a Pokemon using unusualy attacks that that Pokemon shouldn't have, its Mew using transform
Check EVERY room, because Mew can also appear in monster houses (as mentioned above) so if you don't find Mew as any of the red dots you found with the X-ray specs, then it may be in a monster house. Once you find it, and have your friend bow and be on a suitable level, you'll just have to be lucky from then