Leavanny looks like a really cool poke at the moment. The main problem with it though, as others have mentioned is its inability to set-up effectively. It needs a LO to have shot at 1HKO ing 252 / 252 Bold Cresselia, but if it has a LO then it is very susceptible to Thunder Wave paralysis. Not to mention that some Cresselia are now running HP Fire to ruin Escavalier and Durant. I think that Lum Berry might be it's best choice for an item-you still 1HKO wrath after SR at +2, as well as standard 252 HP Rotom, which you would be able to set-up on with a Lum Berry.
As far as partners go, Kabutops is definitely up there. With a Lum Berry, Leavanny is able to set-up easily on Tangrowth and other kabutops counters, while tops is able to rapid spin sr away and beat entei and moltres for leavanny. They cover each others weaknesses almost perfectly iirc.
Spikes and SR also go a really long way in allowing Leavanny to gain some key KO's, such as against max defense Tangrowth. Both Smeargle and Crustle are excellent choices for offensive teams-Smeargle, since it has spore and the ability to almost always get up at least one layer of hazards, and Crustle since it is such a great anti-lead atm. Both are able to get up both stealth rock and spikes. For more defensive teams looking for a way to power through defensive cores, rhydon has great synergy with Leavanny since it resists fire, flying and can smash poison types with it's STAB Earthquake. As for spikers, unfortunately not many synergize that well with Leavanny. However, this often doesn't matter too much as Leavanny will be used as a stall breaker, and it's shared weaknesses with the defensive mons on your team won't matter all that much. My personal favourites are Ferroseed and Roselia-Ferroseed is arguably the best supporter in the entire tier, as with Leech Seed + Spikes + Thunder Wave + Iron Barbs, and with that excellent typing you can set-up spikes on a large number of pokes and quickly wear down physical attackers. Roselia is an excellent counter to things like sceptile and manectric, which can be very problematic for leavanny, as leavanny is going to be taking super effective damage from whichever HP sceptile has, and will fall to an Overheat / Flamethrower from Manectric.
Overall, Leavanny is quite an underrated poke at the moment-however it does suffer hugely from its terrible defensive typing and SR weakness. Underestimate it at your peril though-at +2, Leavanny can decimate defensive and offensive teams alike.
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