I'm sorry if this has been posted before, but I looked around a bit and it didn't looked like it.

Have you ever tried horde hunting ? Whether you were looking for a shiny or a hidden ability, fainting the four little runts before finally being able to catch the one Pokemon you want can be irritating. You have to sit through the horde's attacks, which can take some time, and faint them all one after the other. Painful.

Well, fear no more. With this strategy, you can feint the four horde members you don't want in two turns and get on with the catching ! :)

I came up with it on my own (/u/Lninol, /u/glencurio, /u/undergroundmonorail and /u/grub_step suggested other ideas as well, which I added to this post), and I really hope it can be useful for people. I tested it already and it works wonders :)

The basic strategy

You basically want to pass an immunization ability to the Pokemon you want, and use a target-all attack that goes with the ability. Ideas suggested here are Levitate Earthquake and Lightning Rod/Volt Absorb Discharge. An additionnal startegy is to use Telekinesis Earthquake/Bulldoze/Magnitude for a similar outcome. Sturdy has also been suggested as an ability to be Skill Swapped.

The combos

Levitate Skill Swap Earthquake combo :

Use Skill Swap on a Pokemon, then Earthquake all the others. My original idea. You can use the following to that effect :

• Baltoy / Claydol
  
• Bronzor / Bronzong
  
• Lunatone
  
• Solrock

Unfortunately, they can only learn Skill Swap either by levelling up in Gen 4 or by Move Tutor in B2W2. Since the Bank is out this shouldn't be a problem (for most).

Telekinesis Eathquake/Bulldoze/Magnitude combo :

Send one Pokemon in the air with Telekinesis and faint the others. Credit goes to /u/Lninol for this one.

• Arceus
  
• Baltoy / Claydol
  
• Bronzor / Bronzong
  
• Dusclops / Dusknoir
  
• Gallade
  
• Girafarig
  
• Giratina
  
• Golett / Golurk
  
• Latias
  
• Latios
  
• Lunatone
  
• Metang / Metagross
  
• Mew
  
• Mewtwo
  
• Slowpoke / Slowbro / Slowking
  
• Solrock
  
• Grumpig (Bulldoze only)

I haven't checked them all out individually, so some of them may not be pertinent to X / Y. They all can learn Earthquake and Bulldoze (except Grumpig), so the choice of which attack to use is yours. Golett / Golurk can learn Magnitude as well.

Electric strategy :

Credit goes to /u/glencurio for these. The strategy is similar but does not rely on Earthquake / Levitate. They use the moves Entrainment and Discharge along with their respective Electricity-immune ability.

• Minun (Volt Absorb)
  
• Plusle (Lightning Rod)

Sturdy strategy :

Credit goes to /u/undergroundmonorail for this one. You can transfer the ability Sturdy on one Pokemon and kill the others with an all-target attack. Carbink seems the only viable option here, with Skill Swap and Dazzling Gleam.

This one has the benefit of leaving your Pokemon at 1 HP already.

Trick Earthquake / Bulldoze strategy :

Credit goes to /u/grub_step for the idea. Have your Pokemon hold a Focus Sash or an Air Balloon and Trick it onto the Pokemon you want, and EQ everything else.

Pokemons you can use :

• Arceus
  
• Baltoy / Claydol
  
• Bronzor / Bronzong
  
• Dusclops / Dusknoir
  
• Gallade
  
• Girafarig
  
• Latias
  
• Latios
  
• Lugia
  
• Metang / Metagross
  
• Mew
  
• Mewtwo
  
• Slowpoke / Slowbro / Slowking
  
• Grumpig (Bulldoze only)
  

Again, I haven't checked all of these individually so they may need to be transferred to know these moves. You might notice those are also the same Pokemon that can learn Telekinesis EQ save for Solrock and Lunatone.

There's a risk of losing your item though. Focus Sash disappears on use, and the Air Balloon might pop before you catch your Pokemon. If the Balloon doesn't pop you'll get it back after capture.

Two-Pokemon strategy :

You can also use Chingling / Chimecho with Entrainment and switch out to an Earthquake / Bulldoze / Magnitude user. Credit goes to /u/glencurio.

Exceptions

• Any strategy involving Ground-type moves will not work on Flying-type hordes, such as Hoppip, Zubat, Gligar and Murkrow.
  
• The Electric-type strategy will not work on Ground-type hordes and most likely not on Plusle / Minun hordes with their HA.
  
• All of these will not work on hordes who already have Sturdy as their ability, such as Nosepass, Geodude, Roggenrola and Carbink. You can however use Levitate / Telekinesis and EQ the 'mons twice. Your Pokemon of choice should still be alive.
  
• These strategies will also not work on Zangoose / Seviper hordes as your Pokemon might die before you get to capture it. I'll explain this one in greater details.

For Zangoose / Seviper hordes :

You need a Telepathy Pokemon that knows Skill Swap, and transfer your ability on the odd Pokemon. This will negate whatever attacks the other Pokemon are throwing at it. You'll need to pick off the others one by one.

You can use :

• Elgyem / Beheeyem
  
• Ralts / Kirlia / Gardevoir
  
• Munna / Musharna

Alternatively, if your horde is a 4 Zangooses 1 Seviper one, you can use a Normal-Type Pokemon that knows Synchronoise and use that. This will kill off the Zangooses and leave the Seviper intact. Credit goes to /u/glencurio for this one.

You can use :

• Chatot
  
• Eevee
  
• Whismur / Loudred / Exploud
  
• Hoothoot / Noctowl
  
• Kecleon
  

For these hordes (4 Zangooses 1 Seviper), you can also use Trick-or-Treat on the Seviper, and the Zangooses won't be able to attack it. Credit goes to /u/FilipinoSpartan for this idea.

The only Pokemon that can learn Trick-or-Treat is the Pumpkaboo line.

I really hope this helps, even if just a bit. There might be better strategies, but I wanted to share my own.

If this was already posted somewhere please tell me.

Enjoy :)

Edit : Added one more strategy and info on odd cases.

Edit 2 : Added list of 'mons to use in those odd cases.

Edit 3 : Added the Lightning Rod/Volt Absorb Entrainment Discharge strategy.

Edit 4 : Added Grumpig as a Telekinesis Bulldoze user.

Edit 5 : Added the Sturdy strategy.

Edit 6 : Added more exceptions.

Edit 7 : Formatting a bit spelling.

Edit 8 : Removed the part about Slefdestructing hordes, thanks to /u/FilipinoSpartan for correcting me.

Edit 9 : Added the Trick EQ strategy.