Swampert: WATER, GROUND
Moves: EARTHQUAKE, MUDDY WATER/WATERFALL, FACADE, BRICK BREAK
Swampert is a boss. There is no getting around that fact. So, like any other boss, you need to treat him well. Earthquake is always the obvious choice, especially with his stats and STAB choice. Now, you have a choice: Muddy Water, to reduce accuracy, or Waterfall, just for power. Next, if you are going against anything that is famous for status conditions, you will want Facade for Swampert. Finally, Brick Break just in case if anything happens.
Weaknesses: GROUND, PSYCHIC
Moves: SLUDGE BOMB, ACID ARMOR, MINIMIZE, ROCK TOMB
Muk has been losing the fan favoritism ever since its debut, but I still find it to be very usable in Gen III. Sludge Bomb is great for its poison chance. Acid Armor is awesome, raising his defense +2 every time he uses it. Minimize is the classic annoyance move against enemies without a guaranteed hit move. Finally, Rock Tomb to slow enemies and for variance.
Weaknesses: GHOST, DARK, BUG
Moves: PSYCHIC, CALM MIND, DOUBLE TEAM, SHADOW BALL
Gardevoir is always a fan favorite, and can be used for absolute slaughter against anything not prepared for it. Psychic is an obvious choice, due to its immense power, along with STAB. Calm Mind, if you get the chance to set up, is extremely helpful in laying on the hurt. Double Team is, like Muk, for the classic annoyance. Finally, Shadow Ball for a random Ghost type crossing.
Moves: THUNDERBOLT, DOUBLE TEAM, SECRET POWER, DIG
Most people, when playing through Gen III, choose Manectric, but I usually find Raichu can do the job better. Starting with Thunderbolt is that classic move choice for an electric type, and also helps that it can paralyze. Double Team for annoyance, as previously discussed. Secret Power for general attack, and can still be good. Finally, Dig is for move variance and helps in sticky situations.
Armaldo: ROCK, BUG
Weaknesses: WATER, STEEL, ROCK
Moves: ANCIENT POWER, SLASH, IRON TAIL, SANDSTORM
Armaldo is one of the few rock types that have very few weaknesses, and should be shown off to everyone else. First: Ancient Power is good, mainly due to its random chance at every stat raising. Slash for a raised chance at being a crit. Iron Tail for fighting off Rock types. Finally, if you follow this, Sandstorm is amazing if you set up correctly, especially with Cacturne.
Cacturne: GRASS, DARK
Weaknesses: FIGHTING, FLYING, POISON, BUG, FIRE, ICE
Moves: NEEDLE ARM, FAINT ATTACK, COTTON SPORE, SPIKES
Like most, if not all, grass types, Cacturne has plenty of weaknesses, but can be covered if done right. Before you let Cacturne loose, make sure you Sandstorm with Armaldo do make full use of his ability: Sand Veil, making it harder to hit him. Needle Arm is his famous move, even giving a chance at flinch for the opponent. Faint Attack is awesome due to its inability to miss. Cotton Spore is great at seriously slowing down enemies. Finally, Spikes for anything that isn't flying or levitating, just for entry damage.
EARTHQUAKE Seafloor Cavern - Deepest room near the doorway
WATERFALL Cave of Origin - Gift from Wallace
FACADE Petalburg Gym - Gift from Norman
BRICK BREAK Sootopolis City - Gift
ROCK TOMB Rustboro Gym - Gift from Roxanne
SHADOW BALL Mt. Pyre - 6th Floor
SECRET POWER Slateport/Route 111 - Gift
DIG Route 114 - Gift
IRON TAIL Meteor Falls - Up the waterfall, through first entrance, down the
first ladder, south until steps, down the ladder, on the left
SANDSTORM Route 111 - Desert