Ah, the Frontier. Good times all around. To beat it though, you're going to at least have to EV train and probably IV breed too (though you can definitely survive with the latter).
In any event, a few ground rules should help. First, don't use low accuracy moves unless you absolutely have to (think Metagross Meteor Miss/Mash). Dynamicpunch does nothing for Swampert really, as Earthquake hits almost everything it does harder on average. Swampert is an amazing poke in the Frontier but you're going to want this:
Swampert @ Leftovers
Trait: Pressure
EVs: 252 HP / 120 Def / 136 SAtk
Relaxed Nature (+Def, -Spd)
- Earthquake
- Surf
- Ice Beam
- Mirror Coat / Toxic / Endeavor / Rest
I love Mirror Coat. It's an egg move that is a bit of a pain to get but lets Swampert beat other bulky waters in a pinch 1v1 (think Milotic, Vaporeon, Lapras, Suicune, etc.) Earthquake is your main attack while Surf and Ice Beam round out the set. Also hugely useful for dealing with high BST Pokemon like Dragons, Tyranitar, Metagross, etc.
Castform is terrible. Its stats are terrible, its typing isn't great, it's just horrible.
Torkoal stinks too. It would be better if it wasn't a Fire-type, but you couldn't possibly have worse defensive typing unless they made it Fire/Ice.
Swellow is outclassed by the much stronger Dodrio more often than not. And he is outclassed by Salamence.
Metagross is very good but needs a more focused set:
Metagross @ Choice Band
Trait: Clear Body
EVs: 248 HP / 252 Atk / 8 Spd
Adamant Nature (+Atk, -SAtk)
- Meteor Mash
- Earthquake
- Hidden Power [Rock] / Rock Slide
- Explosion
An absolute terror, it will survive most Earthquakes but STAB Fire moves will hammer it. (Switch in Swampert.) Anyway, I much prefer HP Rock because it doesn't miss, but it's a major pain even for an experienced RNG breeder so Rock Slide is fine (and can flinch slower pokes), the real issue is that you don't have a 100% accurate attack to throw at pokes who absorb Earthquake. Explosion is super useful for dealing with stat-uppers like Snorlax and Suicune who can absolutely sweep teams.
Ludicolo is decent, I'd much rather use Starmie but if you want Ludicolo you probably want:
Ludicolo @ Lum Berry
Trait: Swift Swim
EVs: no idea, but 252 SAtk for sure
Modest Nature (+SAtk, -Atk)
- Rain Dance
- Surf
- Ice Beam
- Hidden Power [Grass] / Giga Drain
Either Grass move is fine. Ice Beam is way better than Blizzard which never hits, especially in the Frontier. The EVs just need to give you enough Speed to outpace Aerodactyl and Jolteon in the Rain. The rest should go in HP.
Anyway my general observation is that you're trying to have this double as a pure in-game team and something that can actually beat the six facilities (excluding the Factory here), but you really have to hone in on the Frontier to beat it. You should have Pokemon that specialize in beating the Frontier who don't need to have to worry about carrying something like Strength, which is a vastly inferior Return. You can get some Silver symbols with in-game pokes (Metagross will bulldoze the Pike Queen's Seviper and Shuckle, for example), but the computer gets too good too fast.
Other useful pokes (accessible in Emerald) in the frontier:
Slaking: the best CBer in town, it kills EVERYTHING
Milotic: sturdy wall if you can catch Feebas
Flygon: decently fast and strong with a great STAB
Salamence: greater CBer but vulnerable to Water-types
Gyarados: the best DDer in the Frontier, it destroys most Waters and a bunch of other stuff too
Breloom: you'll want to rng for hp ghost but he's a monster against wall pokes
Regice: sturdy wall, can be a little underwhelming
Starmie: the best special sweeper you can get
Latios: left it off above because it's probably not rnged, but it's better than Starmie when it is
Latias: see above
Dodrio: just putting this here to mention that Run Away will not help much in the Pyramid so don't waste your time, anyway he's pretty good but outclassed
Heracross: hits like a truck, sadly Megahorn can be unreliable
Make sure to have a defensive presence or you are going to get creamed by any team with decent sweepers once you get to round 4 ish
