Achievements are optional challenges of varying difficulty in the Pokédex that asks you to do tasks in exchange for rewards. In order to get rewards for any achievements, you must first obtain the Pokédex from the Mt. Moon Challenge Level. Afterwards, any achievements you have completed will be recorded in your Pokédex. Claim your rewards and check your progress in the Pokédex.
Have one Pokémon that knows an attack that turns Starmie around so it does not escape. Attacks that can turn Pokémon around are Whirlwind, Roar, and Dragon Tail.. Have an electric-type Pokémon who can use Thunderbolt or Thunder and a Victreebel with Leaf Storm, Solar Beam, or Giga Drain. Starmie spawn on the right path, so having a Parasect with Spore (100% sleep rate) on the right-bottom panel will assure that Starmie is put to sleep.
Another easy way that you can earn this achievement is to use a Pokémon that knows Double Slap on the top-right panel, a Pokémon that knows Bulldoze in the middle, and a Pokémon that knows Whirlwind, Roar, or Dragon Tail to make it turn back on the right-bottom panel. Double Slap can take out some flying types, Magnitude can take out the rest and deals a considerable amount of damage to Starmie, and Whirlwind, Roar, Dragon Tail and Confusion to make sure it does not escape.
You can also try a Pokémon that knows Fissure, Horn Drill or Guillotine. Be sure that the Pokémon's level is higher than the Starmie's level.
You can also use 3 Alakazams using Psychic and two Pokémon from the Diglett family to use Bulldoze. Place the Alakazams in the top left, middle, and the top right. Place the Bulldozers in the bottom left and the bottom right.
You can also use a level 100 Butterfree with Bug Buzz, It works well for all Cerulean Gym achievments.
Also, you can beat the level normally focusing on type advantages. Use this link to know the bosses levels 8 through 45 so that you can use well rounded pokemon by changing their moves based on the boss:
Use four Golem/Graveler with Bulldoze on the four corners closest to the candies. Then use 2 Pokémon that know Psychic on the middle right and middle left to attack the Pokémon from the four corners. Make sure your Pokémon are at least level 44 or above or the Raichus will go past.
You can also get a lot of Ground-type Pokémon with Ground-type moves. Note that you should bring one Electric-type as well and a Flying or Psychic-type for the Tentacools and Machops.
The Hard Way
Defeat Saffron Dojo story level beating the 4th path first, and with your team at level 70 or lower.
Reward: Shiny Hitmonlee or Hitmonchan. (A prize can be claimed up to 3 times per account. Completion required for each claim)
Steel types and Psychic types can resist most of the attacks of the 4th path. For the 1st and 2nd path, any Pokémon level 60-70 will do. For the 3rd path, Gengar doesn't take damage from Fighting-type moves.
Since you need to start on the fourth path, a Pokémon with Worry Seed (Venusaur will do since it's just Worry Seed we're up to) Even if he will faint, it is okay as long as the move is in effect. A Pokémon with Tailwind will clearly boost your speed up and stop your Pokémon from falling asleep. Make sure to bring all your Pokémon out before you use Worry Seed and Tailwind.
A way to do it is bring an Electrode and Clefable, the Electrode should use Protect and the Clefable Minimize. Send in the Electrode and get the candy as far as possible then the Clefable you should get the candy. The third path just send them in they'll get it and the second the same and the first Electrode will solo easily.
The surprise shiny will be either Hitmonlee or Hitmonchan depending on the pokemon you choose when you first complete it. If you chose Hitmonlee you get a shiny Hitmonlee and vice versa.
Defeat Pewter Gym with Pokémon at level 10 or less without losing any candy.
Reward: Shadow Onix. (3 Claimable per account. Achievement only needs completing once)
The best Pokémon available that are around level 8-10 are wild Mankey.
Water-type and Grass-type moves are very useful, as it is 4x effective against both Geodude and Onix.
You can catch a Jigglypuff and teach it SolarBeam,which Grass-type moves are very effective against Rock-types.
Fire Blast is helpful against the Pokémon and deals a bunch of damage.
All the Pokémon in this level are ground-type so Electric-type moves are not very effective.
The Pokémon in this level all have low Special Defense.
You can buy an Abra from the Game Corner and trade it into Mr. Mime then Mr.Mime can do the job easily.
You can't level the Pokémon up over level 10 while doing the level.
You can use a Squirtle with Water Gun, a Butterfree with Confusion, a Pidgey with Gust,and a Rattata with Hyper Fang, level 8-10.
You can also use Four Nidoran M or Nidoran F, all with Ice Beam set as the attacking move.
An easy way to do this if you took Charmander as your starter and you don't have any water or grass attacks, is a Caterpie/Weedle with string shot, and 3 Butterfree using Confusion. Add in a pair of Pidgeys with Sand Attack right at the pokeballs if you want to be extra careful.
A way to finish this challenge is to get 6 level 10 butterfree's all at the entrance for the enemy pokémon, One candy will move, but it won't be taken.
The easiest way is to use 6 missingno.'s at lvl 7-10 and let them use water gun. no candy will be touched
Defeat Cerulean Gym 1 with Pokémon at level 30 or less without losing any candy.
Reward: Shadow Staryu. (3 Claimable per account. Achievement only needs completing once)
A cheesy way to do this challenge is to have two Missingno in the mid-twenties, level-wise, on the two left platforms, a Raichu with Thunder on the central platform, a Level 29 Missingno. on the upper-right platform, and a Paras/Parasect with Spore on the lower-right platform. All Missingno. should be set to Sky Attack, save one which should renew Rain Dance whenever it wears off (to ensure the accuracy of Thunder). The high attack of the Missingno. will demolish everything except for the Starmie, which will be taken down by the Raichu's Thunder.
Defeat Vermillion Gym with Pokémon level 50 or less and without losing any candy.
Alternatively, you can use six Gyarados with Aqua Tail and Earthquake. Set them around the middle with Aqua Tail selected until the Pikachu waves come, when you switch to Earthquake, which should kill everything that comes until the very last wave, when you should change back to Aqua Tail, since it's similar to the first ones, when there are Flying-type Pokémon. Remember that Earthquake hits every single Pokémon in the screen, but the Flying-type ones, such as Gyarados. With six Gyarados about level 38, it is possible to win without losing candy. Just beware of Tentacruel since they use Reflect.
Defeat Celadon Gym with Pokémon level 60 or less and without losing any candy.
Put Clefable in the front spot for each wave with Cosmic Power. Follow Me is optional. Clefable will take 2-4 damage, even from Celebi. Place Fire-type with Fire Blast in the second spot following Clefable. Place Parasect with Fury Cutter in the closest Grass spot (green square) following the Fire-type. Finally put the Pokémon with Helping Hand anywhere (use the resummon/move method to reactivate Helping Hand). Move your team for each wave following this strategy. This strategy is one of the simplest and fastest methods to obtain shadow oddish. The tips below are alternative methods.
Aromatherapy is a very useful move. Although you can't use Vileplume, you still can use Paras or Parasect. If any Pokémon on screen are poisoned, you can just click immediately on your aromatheraphist so it will heal the poison before it hits. Be wary that the timing can be tricky with two giant Muk walking around.
(3 Claimable per account. Achievement only needs completing once)
All you need is a team of Charizards / Aerodactyls / Gyarados / Dragonite with the moves Earthquake, Rest, Rock Slide / Stone Edge and a move of your choice. Place all Pokemons at the bottom (Entei's lair) and select Earthquake. The extras can stay on top. Once Entei does serious damage to your Pokemons, select Rest or if they are almost destroyed by Burns (except Charizard), use Rest then focus on top (Moltres) and use Rock Slide / Stone Edge.
Another strong option is a team of six Aqua Ring users. Options include: Dewgong, Seaking, and Vaporeon. 6 Vaporeons is arguably the strongest. Give them all Aqua Ring, Rain Dance, Hydro Pump, and Helping Hand. The enemy pokemon shouldn't be able to do nearly enough damage to defeat you.
Use 4 Gyrados with Rest, Rain Dance, Thunder and Earthquake. Futhur Info is on the Cinnibar Gym Page.
Viridian City Challenge
Defeat Viridian Gym while not defeating at least 3 enemy from waves 2 to 7.
(3 Claimable per account. Achievement only needs completing once)
Flying-type Pokémon and Snorlaxes with Yawn are good at the start to keep your poke's from getting wiped on wave 8, since you will have 4 attackers coming at you at once. Also need a roar/whirlwhind/dragon tail to turn pokemon around since you need to let three go between waves 2 and 7, but keep them from the candy. I suggest Gyrados (my Lapras was sufficient with it's high health even though it's not flying) with hydro pump and snorlax with yawn, followed by anyone else with a powerful move that hits multiple. A helping hander is a ready must also. Read the Viridian City level list of pokemon to choose which three you want to return on level 8, but be careful with Alakazam since it may be hard to stop him before he gets to come back on wave 8. Let the first two take down Groudon (set attacking to Most Health) while you move the rest of the pokemon of your choice around to finnish off the other three if they survive your first three pokemon. Took only one try.