How about Tail Glow Xurkitree?
Xurkitree @ Electrium Z
Ability: Beast Boost
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
- Thunderbolt
- Hidden Power Ice
- Energy Ball
- Tail Glow
Z- Thunderbolt OHKO Rotom at +0 and Xur can easily tailglow up on rotom since its not really afraid of anything it can do and while it can't safely switch into lando it outspeeds defensive lando and can ohko with HP-Ice.
Not saying that its would be good overall since these teams tend to have Tang or Ferro as well as the prevalence of offense lando or Choice scarf lando overall in the meta but it does beat them both.
Xurkitree @ Electrium Z
Ability: Beast Boost
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
- Thunderbolt
- Hidden Power Ice
- Energy Ball
- Tail Glow
Z- Thunderbolt OHKO Rotom at +0 and Xur can easily tailglow up on rotom since its not really afraid of anything it can do and while it can't safely switch into lando it outspeeds defensive lando and can ohko with HP-Ice.
Not saying that its would be good overall since these teams tend to have Tang or Ferro as well as the prevalence of offense lando or Choice scarf lando overall in the meta but it does beat them both.
[same intro, still stays true]
one of the most threatening pokemon to face, it doesn't appreciate being volturned on but neither of the cores members can come in on it. Rotom-w obviously dies multiple times to a leaf blade and lando can't beat it 1v1 either. Huge threat to volturn teams
while other tapu koko sets find themselves in an awkward spot, shuca koko really appreciates lando-offenses. The whole point of shuca koko is to reliably beat those.
Best switchin to this monster usually is an av-mag on teams with those type of cores. Needless to say rotom&lando die to it. 
the counterpart of kyub. This one has a lot of potential because it not only beats rotom-lando reliably but subs on them and then proceeds to pp stall the countermeasure that was sent in. Roost+sub+pressure has proven a reliable strategy over the course of the generation, while metronome helps building pressure that leftover sets usually lack.
85% chance to claim a kill. Basically every time you hit a fire blast against teams with the featured core. Once rocks are up it performs significantly worse though because being volturned on will result in it being dead. As long as you have a hazard removal this should almost always be an amazing matchup.
While being weak to status zapdos gets great matchups against lando-rotom offenses that rely on kartana/lando to beat electric types. It obviously beats both of those. I'd like to mention defog here since heatwave can be replaced depening on your team and keeping hazards off the field will hurt volturn strategies a lot!
yea well lele is kinda broken against offense and the counterplay is usually limited to switching an avmag into a psyshock once or plainly sacking a mon to then try to revenge it. Pivoting into your dark type on a psychic stab may work as well but honestly this thing is ridiculous to face with a lando-rotom core even though those feature 2 steel types most of the time.
While it certainly beats the core I feel like it is a worse zapdos in this specific situation. Thundi-T only fits on hyper offense, is hard to build around and has a lot less longevity than zapdos. Thanks for posting though, any players that want to build an HO team that struggles with this core will thank you!
Offensive rotom has been a set that gained a bit of attention earlier this year. In this case it performs decently against this core but z-thunder will always trigger the iapapa berry on rotom and doesnt even ko after rocks. After that it relies on hitting 2 thunders. Furthermore the lack of recovery makes it prone to getting volturned on and you usually don't get a pain split off in these matchups. Thanks for suggesting though, it's important to have it listed here.
yea sure it probably beats this core but it's not a good set and is too cteam-y to consider it for a good team. I've tried shuca magearna myself and while it can work, it fails to work in just as many situations and most teams that feature it are lacking something important which leaves them with no choice but to do some role compression. Especially this set can't do what it's supposed to do without the fairium z.
This suggestion feels a lot like the thundurus-t suggestion, but it even lacks z-hypnosis. You already mentioned that it's not good overall and as you said it DOES beat the core of two. It's not reliable enough to make it a threat of the round but thanks for posting it!
Round 7 Core: Diancie + Volcarona 
@ Choice Specs 
the all-feared specs ash but protean sets with water coverage leave this core no chance bar hydro pump misses
While it obviously needs rain to 1v2 both easily, it even claims a kill w/o rain and prevents set-up. And no, diamond storm is the exact move it needs. I also don't want to lose my entire bulk just to have better endeavors against chansey, when endeavor hurts it extremely nonetheless.
while endeavor diancie + volc beat it 2v1, tox can always switch out after endeavor, assuming there are teammates left. Annoying to say the least
av tar beats both 1v2 if it doesn't get crit and can counter 1 even if it comes in. Also offers opportunity to use excadrill which is another tool to beat this core.
banded azu really isn't a set in gen7, z-belly azu would get TotR though since both members can't KO it so azu easily gets it's health back and blows our core to hell
diancie does tons of damage with it's stabs, especially diamond storm and volcarona has potential to set-up on it if it's chipped. Lives a few surfes as well due to spdef boosts and pinch berry