Bro just going from Gen III Pokémon.
Swampert-Ground and Water type with good attack and HP and a Mega Evolution. It fits into a lot of roles and is pretty hard to deal with. Earthquake, Scald, Stone Edge/Sludge Wave, and Hammer Arm give it a pretty well rounded learn set.
Shedinja-Give it a Focus Sash. It only has one HP and the Focus Sash prevents a one hit KO thus giving you a free turn. Just can't use it when there are physical hazards on the field.
Salamence- Seriously. This thing will eat souls.
Gardevoir- Beast special attack and if you give it Protect and Recover then you've eaten up two of the three turns required to survive until Future Sight comes into play to give you an extra hit alongside Psychic. Not to mention Future Sight still hits after Gardevoir is knocked out so you can sacrifice it to take out the really tough opponents.
Metagross- Ignoring its already gaudy stats. Bullet Punch for an increased priority move. Earthquake of course. Psychic for those pesky flying types. Then there's always Explosion for a last resort.
Manectric- Since its the best of the abysmal Gen III electric types. Boost its speed and attack. Volt Switch so you can always swap it out and keep the type advantage not to mention a quick way to use Toxic for a poison or Thunderwave for the paralysis and then deal damage on your switch out for a type advantage. Then give it a big special attack move like Overheat.
