Please don't use aqua tail unless you don't have waterfall.
Waterfall has 100% accuracy, a 20% chance to flinch, and is only 10 damage less than aqua tail. That 20% chance to flinch will save your life, believe me. Also waterfall is an HM that is useful outside of battle. Lastly, waterfall has 50% more PP than aqua tail.
Also, what kind of Swampert you want to use can change this a lot. M-Swampert has swift swim while the regular one has torrent. If you're going Mega form, then you might as well pack rain dance or have a drizzle user on your team for the bonus speed.
Swampert's two main STABs are waterfall and earthquake though sometimes you might see a more wall-ish Swampert use something like scald for the burn chance.
Other common offensive moves include:
1. Stone Edge - Edgequake combo is the one of the strongest coverage in the game.
2. Ice beam - Usually used when the person is running scald.
3. Ice punch - For coverage.
4. Hammer arm - Somewhat useful though the coverage is kind of poor.
Other common non-offensive moves include:
1. Stealth rock - Its uses are endless and makes a huge portion of the roster useless.
2. Roar - Swampert is great at phasing out threats due to its typing and bulk.
3. Toxic - Toxic is always great on bulky pokemon.
4. Curse - Swampert is much like Snorlax in this regard, he can slowly bulk up while sacrificing his nonexistent speed especially if you don't want to swift swim.
5. Wide guard - Used a lot in multi battles.
Honestly I think Swampert only needs earthquake and waterfall to be at its best. Anything else is just gravy. I have swept entire teams with rain dance swift swim M-Swampert too. I'm partial to this build:
Swampert @ Swampertite
Ability: Damp
EVs: 248 HP / 252 Atk / 8 SpD
Adamant Nature
- Waterfall
- Earthquake
- Stealth Rock
- Rest
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Cthulhu's advice on Breloom is good, use that.
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Manectric only has a use as a megavolution though sometimes rarely you'll see him used in multi battles for his lightning rod ability. I guarantee that it will be choiced if it isn't mega-evolved.
The build is usually something like this:
Manectric @ Manectite
Ability: Lightning Rod
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Volt Switch
- Thunderbolt / Thunder / Discharge (multi battle)
- Overheat / Flamethrower
- Hidden Power [Ice] / [Grass]
In multi battles you might see it run trick because it will be choice spec-ced.