Whether you use a physical or mixed Swampert, your only really good options are earthquake, return, surf, and ice beam. Moves like brick break and rock slide are weaker than a STAB earthquake even when they're super effective, inaccurate moves like hydro pump and blizzard are dangerous to use in Battle Frontiers, and hyper beam and focus punch are even less reliable. Here are some calculations for ground resists that you might encounter in your playthrough or the Battle Frontier. (assuming no EVs on Swampert and torrent isn't active)
A special attack does more damage to these Pokemon.
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Shiftry: 70-83 (35.7 - 42.3%) -- guaranteed 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Shiftry: 105-124 (53.5 - 63.2%) -- guaranteed 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Cacturne: 70-83 (40.6 - 48.2%) -- guaranteed 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Cacturne: 105-124 (61 - 72%) -- guaranteed 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Altaria: 51-61 (28.6 - 34.2%) -- 2.5% chance to 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Altaria: 136-160 (76.4 - 89.8%) -- guaranteed 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Flygon: 56-67 (30.4 - 36.4%) -- 54.1% chance to 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Flygon: 170-200 (92.3 - 108.6%) -- 56.3% chance to OHKO
-1 Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Salamence: 38-45 (18.8 - 22.2%) -- possible 5HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Salamence: 170-200 (84.1 - 99%) -- guaranteed 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Dodrio: 62-74 (38.7 - 46.2%) -- guaranteed 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Dodrio: 105-124 (65.6 - 77.5%) -- guaranteed 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Weezing: 40-48 (24 - 28.9%) -- 99% chance to 4HKO
Lvl 60 0 SpA Swampert Surf vs. Lvl 60 0 HP / 0 SpD Weezing: 69-82 (41.5 - 49.3%) -- guaranteed 3HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Crobat: 56-67 (29.4 - 35.2%) -- 17.3% chance to 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Crobat: 85-100 (44.7 - 52.6%) -- 25.8% chance to 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Xatu: 62-74 (37.3 - 44.5%) -- guaranteed 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Xatu: 93-110 (56 - 66.2%) -- guaranteed 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Skarmory: 17-21 (10.2 - 12.6%) -- possible 8HKO
Lvl 60 0 SpA Swampert Surf vs. Lvl 60 0 HP / 0 SpD Skarmory: 69-82 (41.5 - 49.3%) -- guaranteed 3HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Sceptile: 66-78 (38.3 - 45.3%) -- guaranteed 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Sceptile: 79-94 (45.9 - 54.6%) -- 59% chance to 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Breloom: 56-67 (35 - 41.8%) -- guaranteed 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Breloom: 105-124 (65.6 - 77.5%) -- guaranteed 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Claydol: 45-53 (28.1 - 33.1%) -- guaranteed 4HKO
Lvl 60 0 SpA Swampert Surf vs. Lvl 60 0 HP / 0 SpD Claydol: 91-108 (56.8 - 67.5%) -- guaranteed 2HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Latias: 51-61 (27.7 - 33.1%) -- guaranteed 4HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Latias: 57-68 (30.9 - 36.9%) -- 73.3% chance to 3HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Latios: 56-67 (30.4 - 36.4%) -- 54.1% chance to 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Latios: 64-76 (34.7 - 41.3%) -- guaranteed 3HKO
Return does more damage to these Pokemon.
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Ludicolo: 62-74 (33.6 - 40.2%) -- guaranteed 3HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Ludicolo: 35-42 (19 - 22.8%) -- possible 5HKO
-1 Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Gyarados: 38-45 (18.8 - 22.2%) -- possible 5HKO
Lvl 60 0 SpA Swampert Ice Beam vs. Lvl 60 0 HP / 0 SpD Gyarados: 35-42 (17.3 - 20.7%) -- possible 5HKO
Lvl 60 0 Atk Swampert Return vs. Lvl 60 0 HP / 0 Def Heracross: 59-70 (32 - 38%) -- 95.1% chance to 3HKO
Lvl 60 0 SpA Swampert Surf vs. Lvl 60 0 HP / 0 SpD Heracross: 54-64 (29.3 - 34.7%) -- 8.8% chance to 3HKO
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As you can see, you'll want to use either surf or ice beam against most of the ground resists in the game, partly because Swampert gets STAB with surf, ice beam just happens to be super effective against a lot of ground resists, and Swampert's base special attack really isn't that much lower than its base physical attack. This is the moveset that I recommend for the Battle Frontier.
Swampert @ leftovers
EVs: 100 HP
- earthquake
- surf
- ice beam
- protect
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