Once finished the first Island Trial, wild Pokemon will then be able to call upon other pokemon during battle. Not all pokemon call allies, and not every pokemon calls it's species as an ally. When a pokemon calls an ally to battle it will be a two on one situation. I like to call the Pokemon on the right the 'caller', the caller will only call one pokemon into battle, so if you want to chain this pokemon you need to use an adrenaline orb to allow the caller to call more pokemon and increase the odds. The fastest way to get the caller to call more pokemon is for it to have 1 HP, so false swipe pokemon are your friend here. Once the caller is at 1 HP, you use the adrenanline orb, all you need to do is faint the 'called' pokemon, one after another, thus starting the chain. Every Pokemon fainted is incremmenting the chain, but before we get to the benefits of chaining theres one important reminder.
There are two main ways to chaining pokemon
Certain Pokemon knows moves like mimic, where they will copy a move you use on them. For these Pokemon you can bring in a Pokemon that knows Recycle and spam it until the opposing Pokemon uses Mimic. Once they know Recycle you can bring in a Pokemon that knows Switcheroo and holding a Leppa Berry. This will transfer the Leppa Berry to the 'Caller' Pokemon. Once they run out of moves they will have to use Recycle and it will refresh the PP of one of their moves thus making sure it never struggles. You need to do research on what Pokemon know Mimic or you can transfer Recycle too. In the future I will provide and updated list of who those Pokemon are.
If a Pokemon is at 1 HP and runs out of moves, they will try to struggle and faint themselves, so you need to come up with a count in your head for when to switch the caller. Some Pokemon like chansey have light screen which is a 35 pp move so they can be in battle for awhile, but other Pokemon might max out at a total of 30-40 PP. To be safe every 10-20 pokemon you faint consider swapping the caller, to do this simple take the new called pokemon down to 1 HP, and fainting the orginal caller Pokemon. DO NOT, apply status condtions to a Pokemon as it can make them calling a Pokemon into battle either non exsistent or the rate is extremely reduced. Only use status conditions when you have the Pokemon you wan to catch. So now that we are passed the intial setup lets go into detail.
The key number here is 30, a chain of 30 Pokemon fainted starts to provide insane benefits. While there are small increases in these benefits up to 30 I will not be covering them.
If you want to EV train your Pokemon, this is hands down the fastest method ever in Pokemon. Buying items doesn't count because most games take awhile to acquire money which most people do not consider.
So to get 252 EVs in one stat each method above works as follows
I would also like to mention every Pokemon in your party who gains EXP from a battle [EXP share] takes part in gaining EVs points. So if every pokemon in your party had the associated held item and Pokerus, it would take 7 fainted wingulls to increase 6 Pokemons Spd EV points to max. making it the offical fastest EV training method out there. Beside owning x999 of each EV increasing Item which in my mind doesn't count.
Call rate 3 - Base: 0.36% - Max 30%
Call rate 9 - Base: 3.24% - Max 90% Mimikyu is a tricky pokemon because its ability Disguise, which allows it to get hit without taking damage the first time. For this shiny hunt specifically we want a Adamant/Jolly nature so we will still be utilizing sycnhronize. Umbreon is the go to here as it can learn substitiute and have scynronize. The reason for susbsititue is Mimikyu likes to use Charm to lower your attack and makes it harder to false swipe. Then you need to spam 2 moves per Mimikyu until you get a shiny.
The initial setup here is to use a pokemon that know Recyle until Mimikyu uses Mimic. Once Mimikyu uses Mimic on recyele it will take the move from you. Then we swap in a pokemon that knows switcheroo holding a Leppa Berry, thus giving Mimikyu a Leppa Berry to hold for when it runs out of moves. Then you can bring in a Pangoro with the ability Scrappy and knows False Swipe, as this allows it to be used on a ghost type pokemon.
Now that the setup is complete we swap to Adamant/Jolly nature synchronize Umbreon and use substitiue. Umbreon should be holding leftover for constant health regen. Now just spam Crunch over and over until you get a shiny Mimikyu.
Once you get a shiny Mimikyu, in order to be safe, you are going to run the original setup using receyle and another pokemon with switcheroo. THIS IS NOT NECESSARY but if you can't catch it in time you lost out. Now bring in pangoro and false swipe it down. If you have another pokemon with yawn/hypnosis etc... put it to sleep to make for easy catch. I know alot of people like paralysis so it can't call anymore pokemon and thats fine too its all just preference. It also can't call while asleep.
Note: On turns when help is not called in use a Leppa Berry or Double Team/Work Up to make the chaining easier. Umbreon also learns Yawn if you want to put it to sleep while you swap out.