I've seen ridiculous amounts of Tangrowth, as well as Slowking. However, Slowking has always had high usage, so it's always something that you've had to prepare for extensively. However, Tangrowth has only become popular recently, so it's interesting to see how the metagame adapts to it. Tangrowth has always been good, it's just that now people are starting to see it. With Slowking it forms the infamous Regenerator core that is starting to take over the tier. I swear, if VoltTurn is OU's thing, then the Regenerator core is RU's (TangKing or SlowGrowth, anyone?).
I've been using Magmortar to combat this, and it's probably the most anti-metagame Pokemon right now. It can switch into Tangrowth with ease because of Vital Spirit, something which Moltres lacks. Honestly, one of the toughest things about switching into Tangrowth is that it can remove a lot of its counters from the game with a simple Sleep Powder. I think that Expert Belt Magmortar is the best set, since it doesn't punish misprediction while easily bluffing a Choice item. It is better than Life Orb in my opinion because giving up Magmortar's good special bulk is less than ideal. Also, its weakness to Stealth Rock combined with Life Orb recoil hurts its longevity tremendously, and its ability to consistently switch in to Tangrowth is one of its best traits.
Another perk of using Magmortar is Thunderbolt. Modest Expert Belt Thunderbolt is a guaranteed 2HKO on 248/112+ Slowking after Stealth Rock, making what is usually your opponent's only Magmortar switch-in unable to stop it. Therefore, Magmortar is also an excellent lure for Slowking, allowing Moltres to sweep if Magmortar can't get the job done by itself. As shown by Manectric, Fire/Electric coverage is just really good in general in RU, but Magmortar has even more coverage options. Focus Blast is a great tool that 2HKOes Clefable and Lickilicky, making the list of Pokemon that can switch into it even shorter. Even Munchlax is taking 42% minimum from Focus Blast, and if its asleep is completely useless against Magmortar. HP Grass rounds out the coverage, OHKOing Sandslash after one layer of Spikes as well as Rhydon. Lanturn also takes around 45% from HP Grass, and its lack of reliable recovery means it can be easily worn down to the point where Magmortar can power through it.
Overall, people are going to start to have to find counters to the Slowking + Tangrowth Regenerator core, and Magmortar is a fine example. Magmortar should just be used more in general, and I expect it to move to RU in the next tier shifts.