This is a pretty fun team that i built around with the newly released mega swampert. i feel like all these balance teams running around with stuff like landorus, toxapex, tangrowth, zygarde, scizor, zapdos etc. all get heavily dented by mega swampert. absolutely love this mon. lets go over the individual members.
Pelipper is my rain setter as usual. the EV spread is chosen here to capitalize on its decent defensive typing, checking things like scizor and non tpunch zard x. i think the thing that made pelipper so valuable over politoed is its reliable recovery in roost and a way to pivot into rain abusers with uturn.
Mega Swampert is, like previously mentioned, a threatening sweeper under rain. not only that, it has the bulk to take hits such as tapu lele's psychic, specs surf from kingdra under rain, and dark pulse from ash greninja. the typing also is very nice, allowing you to naturally check tapu koko, thundurus, and non-bloom doom heatran. the EV spread is pretty self explanatory: power and speed. a jolly nature can be run to have enough speed to outspeed scarf terrakion and garchomp, but adamant is generally preferred. the last move is pretty flexible. superpower does about 60 min to ferrothorn, stone edge lets you hit pelipper and mantine, and rain dance can be used as a last ditch effort if pelipper dies. alternatively, but definitely more situational, if you are finding yourself weak versus tapu bulu, you could even run sludge wave, which does 88 min. (ice punch only does about 60 to bulu.)
Kingdra is the secondary rain sweeper that pairs well with mega swampert due to their physical/specially offensive synergy. EVs are self explanatory. kingdra helps break down things that may annoy swampert, like rocky helmet landorus t/garchomp, and also helps weaken shared checks like ferrothorn and tapu fini, both of which don't have reliable recovery (if ferrothorn isnt running leech seed.) in return, swampert beats down chansey. kingdra also gives a check to opposing rain, because it can come in on a kingdra locked into surf or do upwards of 90% to swampert.
Ferrothorn, i find, is about as equal to kingdra as being a necessity on a rain team. it just brings so much to the table in this metagame, especially on rain. first off, rain gives ferrothorn basically an 8-turn occa berry, which is great to assist in its longevity. also it gets up spikes which greatly helps the team for pokemon like kingdra and swampert to click their stab moves with ease. three water types on the team means they share similar weaknesses, and ferrothorn covers them with its phenomenal typing. it checks things like lele, ash greninja, koko, landorus-t, etc, setting up spikes and annoying the opponent with leech seed, and in turn, the rain core obliterates things ferrothorn is weak to, like heatran and magnezone.
Volcarona is an interesting pick i threw on the team originally with hurricane but decided to switch it up. volcarona gives me a backup win condition if the opponents team has been dented heavily by rain pressure, spikes, scald burns, and leech seed residuals. bug buzz is chosen here because i feel that with 3 water types and knock off from clefable, heatran isnt that much of an issue for me to run hp ground on volcarona. bug buzz is less commonly seen this generation, but it allow volcarona to muscle through some of its regular checks like latios, mega gyarados, tyranitar, and also go through potential substitutes. HP ice can also be run allowing you to hit zygarde harder at +1, but unless it is weakened, it's just going to toxic you or hit you up with a banded thousand arrows. psychium z allows volcarona to break through the likes of common rain annoyances, like toxapex, mega venusaur, and even the rare tentacruel. fiery dance is a more accurate stab that will probably be more useful late game, but fire blast can be used as well for the extra power.
Clefable is a very solid mon i wanted to put on the team, as it gives me a check to so many mons, like choice-locked gengar/keldeo/garchomp etc. knock off is incredibly useful on this set, because it is able to get rid of so many important items like marowak's thick club, tangrowth's assault vest, heatran's leftovers, toxapex's sludge, and so on. clefable is always an annoyance due to its bulk and typing, and the fact it gets recovery. healing wish is also crucial on this set because it gives a second life to any one of the mons which can still put a heavy dent in the opponents team when healed up. it essentially patches the team together with its versatility.