Molk
Godlike Usmash
You'll probably be seeing Amoonguss used over Tangrowth most of the time, but the point still stands that these cores can be quite annoying. However, they aren't impossible to deal with.So I've been running into a ton of Slowking, Tangrowth, and Moltres, as I'm new to RU it has been causing issues, only one loss so far but still far to close. Any tips to get around/destroy those three if they are on the same team?
Life Orb Druddigon is a good Pokemon to help out with the Regenerator core to start off. All 3 Pokemon take heavy damage from Outrage, and even Tangrowth gets cleanly 2HKO'd by a Life Orb Sheer Force boosted Gunk Shot. It doesn't really have any trouble switching in on either member of the Regenerator core either as long as Amoonguss's Spore has already been used up.
Calm Mind Psychic-types such as Reuniclus, Cresselia, and Sigilyph do pretty well against this core too. Once Spore's been used up they can also set up on and defeat both members of the Regenerator core (bar dtail Slowking), and they can break through afterwards. In general, Psychic-type Pokemon tend to do well against RU's defensive cores.
Exploud is another Pokemon that works well. just switch it in with Stealth Rock up and neither Pokemon wants to take repeated Boombursts, even Assault Vest Slowking is taking over 40% from Exploud, and it won't be able to do enough to KO in return, not to mention Stealth Rock pushes AV Slowking into 2HKO range regardless.
In general, Pursuit+a Pokemon that appreciates Slowking removed is a good way to combat this. Pursuit puts Slowking in a checkmate position, preventing it from safely switching into its partners and possibly taking it out right then and there, clearing the way for the Pokemon in question. Good users of Pursuit include Spiritomb and Escavalier w/ some Speed creep, and Pokemon to pair with these Pursuit users include Moltres and Delphox.
hope i helped!