Tangrowth @ Leftovers / Rocky Helmet
Ability: Regenerator
EVs: 252 HP / 252 Def / 4 SpD
Relaxed Nature
- Sleep Powder
- Giga Drain
- Knock Off
- Hidden Power Fire / Hidden Power Ice / Leech Seed
Slowking faces competition from Slowbro. The main advantage Slowking has is the higher special defense to take on special attackers better. However, the loss in physical defense means Slowking takes hits from physical attackers less effectively than Slowbro does. Examples of these attackers include Exca, Chomp, Lando-T, Mega Lop, Terrak, and Azu. This is where physically defensive Tang comes in to help cover these Pokemon, making up for some of the threats you handle less reliably by using Slowking over Slowbro. Further, physically defensive Tang helps Slowking by dealing with other troublesome Pokemon like Loom, Mega Gyara, and Tar.
With no AV, Tang has a harder time taking hits from Pokemon like Keldeo, Mega Diancie, and Mega Zam. Slowking helps with this by making use of its solid special defense. Slowking also assists by checking several other threats Tang doesn't like running into, such as Volcanion, Tran, Talon, and CM Clefable.
These two have your classic and solid Grass + Water synergy overall, and form a nice and durable Regenerator core capable of dealing with a good number of threats. I think Slowking does a nice job using its main advantage over Slowbro (higher special defense) to cover Tang well. Similarly, Tang does a good job covering the loss in physical bulk. Although Tang doesn't do much to help Slowking with threats like Serp, Mega Sciz, and Mega Manec, other solid Pokemon like Tran can easily be used with this core to help with these issues.
Regarding item and move choices on Tang: assuming you don't mind ditching Leftovers, Rocky Helm could be a good choice to punish U-turns from Pokemon like Lando-T and physical attacks aimed towards Slowking in general. HP Fire can be used to give Pokemon like Mega Sciz and Ferro less opportunities to annoy the core, while HP Ice is also viable to damage offensive grounds like Chomp and Lando-T harder.