Mega-Z Ring or DJ Arclight Sidequest is one that can get a bit confusing and all people have stuff that you need for the other people so you need to do a bit of everyone at a time.
7th Street Edit
At 7th Street go to the South-East corner and buy all the pokemon there while leaving DJ Arclight will appear saying how those Pokemon are stolen and how you need to rescue them, if you bought them all he'll say how incredible it is and how that guy will be out of business. He will also reward you a Lucky egg for your troubles.
After that it'll be a while until you can talk to him again, to continue the sidequest you will have to first defeat Gym Leader Titania after you've done that go to the Opal Ward.
Opal Ward Edit
In the Grand Hall DJ Arc will be in the South-East Corner of the area just right of the entrance, when you talk to him he will say about how you need to get Team Meteor for the stuff they did and how he has been doing some research on the owners of the stolen Pokemon and that he found 5 of the trainers who had their Pokemon stolen and that you only need to do the deliveries, by talking to him again he'll give you the list of who needs their pokemon back:
- Lillin from the Jasper Ward lost a Loudred
- Felicia from the Apophyll Academy lost a Makuhita
- Mr.McKrezzy from the Lapis Ward lost a Roggenrola
- Santiago from the Peridot Ward lost a Jigglypuff
- Eustace from the Lapis Ward lost a Vanillite
Santiago will be in the Peridot Alley
After you go into the alley Santiago will be the person closest to the entrance when you talk to him he'll seem ruff at first but then he'll tell you how he loves Jigglypuff and that he doesn't want the others to find out or they'll make fun of him and ask you to give the Jigglypuff to some other person in the meantime and to tell him who you gave it to, he will then battle you so the others dont get suspicious he will have 3 Pokemon:
- Azumarill Lvl83
- Lopunny Lvl86
- Sylveon Lvl84
The person that will take the Jigglypuff is Lillin whose part of the quest will need to be completed before you can talk to Santiago again.
After completing Lillins side of the quest go back to the alley and talk to him to tell him Lillin has his Jigglypuff he will then thank you and say that if you ever need anything you can just ask.
Lillin will be in the Jasper Ward in the building closest to the south entrance to the Malchous Forest, when you talk to her she'll mention about how after all this time her piece has finally come to an end and asks if she really has to take it back and how it is so hard for her to sleep with Loudred around, after talking to her a second time she will agree to take Loudred back, by talking to her a third time you can give her the Jigglypuff and she will then have an evil laugh and say that the Jigglypuff is now hers (even though the guy would 100% BODY her if she were to refuse giving his Jigglypuff back) after a bit of talking she will reason that she will only keep Jigglypuff until you can get her some way to tolerate its screeching which will be the Noise Cancelling Headphones Felicia from Apophyll has.
After completing Felicias side of the quest and getting the headphones go back to the Jasper Ward and talk to Lillin to give her the headphones she will then try them on and ask you to say something a choice will then appear which does not matter, she will then tell you to tell the guy he can come get his Jigglypuff anytime.
First go to the Lapis Ward building left of the Pokemart inside you will see a woman, she will then talk about how her son has been kidnapped and that she at first thought he was with her brother on Spinel Town but a bald-headed guy just gave her a ransom note, she will then explain that her brother is a famous musician called Master McKrezzy and that her son was kidnapped to pick at McKrezzys success and to please save his little Eustace.
After doing Santiagos part of the quest go to the Peridot Alley and talk to Santiago to ask about Eustace he will tell you that he indeed has him, he will then say to not get him wrong he doesn't approve of fucking around with kids and that this situation is a little different he will tell you that you'll see it when you get there and tell you to go to one of the roofs of the Onyx Ward buildings and to go fuck him up.
Go to the Onyx Ward, the building in question is the one left of the Casino (careful because you will need to battle twice without healing), enter the building and go to the roof, as soon as you step out a bald-headed guy will ask what the hell you're doing there and another will ask how you found out about them, leading to a double battle with them having 6 Pokemon:
- Weezing Lvl82
- Muk Lvl82
- Tsareena Lvl83
- Zangoose Lvl85
- Mudsdale Lvl83
- Lickilicky Lvl85
After defeating them the boss of them will reveal himself to be indeed Eustace and proceed to insult you and them and proceed to explain the plan, after that he will battle you, he has 5 Pokemon.
- Manectric Lvl86
- Alakazam Lvl90
- Sharpedo Lvl88
- Sceptile Lvl87
- Scizor Lvl90
After defeating him he will fire the goons and head home, go to the Lapis Ward and talk to the mom she'll tell you some small stories of what he did before, after that go upstairs and talk to Eustace he will tell you about why he did the scheme to get his uncles money, talk to him a second time after he tells you to give him tons of lvl160 Electrodes give him his Vanillite back he will then realize the error of his way and tell you that he'll try to be a better person who does good things.
Go to this building in Spinel Town
After going inside talk to him he will tell you he doesn't do autographs and that he is here on a much needed vacation, talk to him a second time and give him his Roggenrola he will then say that you have his Roggenrola and how thankful he is about it and will apologise for being so dismissive before.
After getting his autograph for Felicias side of the quest you can talk to him again to fight him for a cool reward he will have 6 Pokemon:
- Incineroar Lvl83
- Starmie Lvl85
- Staraptor Lvl87
- Omastar lvl86
- Flygon Lvl86
- Armaldo Lvl85
Reward: 6785 Pokedollars and TM15 Hyper Beam
She will be in the Apophyll Academy middle left room, and there is a Purple Shard in the rock.
When you talk to her she will say that she can't hear anything through the headphones and will ask if you need something she will then get excited because she has been looking forward to the moment you arrive to ask you for DJ Arcs autograph.
Go back to the Grand Hall in the Opal Ward and talk to DJ Arc again he will get a little embarassed and give you the autograph.
Once more go back and talk to Felicia to give the autograph to her, she will get excited and thank you after that talk to her a 3rd time and give her the makuhita back.
And just like a broken record talk to her again, she will tell you that her headphones are precious and the only thing she would trade them for is Mr.McKrazzys autograph.
After completing Mr.McKrazzys Side of the quest speak to him again he will then give you his autograph.
Go back again and talk to Felicia to give her McKrazzys autograph she will happily give you the headphones.
Go to the Grand Hall in the Opal Ward and talk to DJ Arc and he'll tell you that it's a shame about the other trainers he wasn't able to track down and that it's up to you to take good care of the remaining Pokemon.
Your Reward will be the Mega-Z Ring with it you'll be able to use Mega stones and Z-Moves