Could someone please tell me how to catch Shedinja, Feebas, Nosepass, ..

Shedinja - Train a Nicada up
Feebas - Change the groovey saying in Dewford and then fish around
Nosepass - I don't know
Koffing - Find one
Mudkip - Starter Pokemon - have to trade
Treeko - Starter Pokemon - have to trade
Aggron - Get a Arron and train it up
Machamp - Train a Machop up into a Machoke then trade Machoke - Machamp
Chimeco - Catch it on Mt Pryer
I am just trying to help...

Shedinja:
Train nincada to level 20 with at least ONE extra space in your party.

Feebas:
Go to dewford town change the trendy saying and fish from Mauville city to LilyCove City.

NosePass:
Have a pokemon with rock smash go deep into the cave on dewford use rock smash he/her may pop up.

Koffing:
Find one in the cave from Fallarbor town and Mauville City.

Mudkip:
Ar or trade one

Treeko:
Ar or trade one

Aggron:
Catch a aron in dewford or victory road and if your lucky you may find a Lairon in V Road.

Machamp:
Trade machoke and trade it back

Chimecho:
Catch one at mt.Pyre it is rare it took me 20 to 40 hours just kidding minutes

OK, I'll go Pokemon-by-Pokemon.

Shedinja: When Nincada evolbes into Ninjask at level 21, have a spare space in your Party, and a copy of the ball you caught Nincada with in your bag. When Nincada has finished evolving, you will see that Shedinja has mysteriously appeared in your party.

Feebas: You can only catch Feebas in the river of Route 119. I classify Feebas as the most hard-to-find Pokemon in the game, because they aren't very common. In fact, Feebas is only located in 6 tiles out of the 400 in the river, making it very hard to find, as they only appear in certain parts of the river. I found it by going to every spot in the river from the bottom and using Old Rod. Eventually I found one. Make sure you use an old Rod. If you find one it'll be on about level 26. Here's some good news. In the tiles that Feebas is located in, you have a 75% chance of finding it in there when you fish for it. Make sure you're patient. And the places where Feebas is in the river are different for each game, so dont get a friend to tell you where they found theirs. Also, if you change the Dewford Town saying, Feebas changes the tiles that it's in, in the river for some strange reason. To evolve Feebas, max out it's beauty stats, by feeding it Blue Pokeblocks, then raise it up one level. It will evolve into Milotic, one of the longest and the most beautiful Pokemon in the game, unlike Feebas. It's kinda like Magikarp and Gyarados.

Nospass: You can only find Nosepass, by using Rock Smash on some of the breakable rocks in Granite Cave. Most of the time, you'll find nothing, or Geodudes. But if you're lucky, you'll find a Nosepass.

Koffing: Koffing can be found only in Fiery Path. If you are using Sapphire, they are very rare, but Grimers are common. If you are playing Ruby, it's the other way around. Koffing evolves into Weezing at Level 35.

Mudkip: Mudkips can be obtained three ways. One, get it as a starter Pokemon from Professor Birch at the start of the game. Two, get it from a friend in a trade, who has a Mudkip. Three, get one of your friends to trade you a female Mudkip, Marshtomp or Swampert. Then breed it with another Male type. If you have one of the right Male water Pokemon, you will get an Egg from the Dyacre man after a while. Walk around heaps, and the Egg hatches into Mudkip.

Treecko: Same as Mudkip, execpt for number three. What you have to od is get a female Treecko, Grovyle, or Sceptile from a friend, then breed it with one of the right Male grass-types. The rets is the same, except that you're egg hatches into Treecko.

Aggron: First find a wild Aaron anywhere in Granite Cave. Raise it to Level 32, and it will evolve into Lairon. Raise it a further 10 levels to 42, and it will evolve into Aggron.
Machamp: First, find a wild Machop, anywhere on Route 112, or in Fiery Cave, Raise it to level 28, and it will evolve to a Machoke. Then use must trade Machoke to another game. Then it will evolve into Machamp on the other game. If you want your Machamp, you have to trade it back. This is also the process you use to get a Golem, Alakazam, or a Gengar (Only in FireRed and LeafGreen versions).

Chimecho: You can onlt get Chimecho in the grass of the outisde area of Mt. Pyre. To access this area, when you walk into Mt. Pyre, and you're in the first room, walk through the door in the bottom-left corner of the room. Keep walking North outisde, until you find an area covered with fog. This is where you are most likely to find Chimecho, howver, it is still extremely rare. Also, at the very top of this outside area, you will find the home of the Red and Blue orb, and two people guarding it, who can tell you stories about them.

Nosepass can be found in granite cave and will appear when you break a rock with rock smash although this may take a few trys