Mega Gengar
I would generally go neutral Speed natures on base 90 scarfers unless my team is a bit weak to another base 90 scarfer. On Kyurem, I would think a Timid nature would be better just to outspeed +1 Adamant DD Rayquaza (they run Adamant these days to have wall breaking potential when they can't set up) above all else. Although these are rare, they can run over teams in a heartbeat. If you are running Scarf Kyurem, you might as well use it as a reliable way to revenge KO this monster since it is definitely able to take a +1 E-Speed and already has absurd Special Attack to begin with.
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Many of the lower ladder players of our tier (I'm just generalizing here so not everyone falls under this) are just casual players who don't care for competitiveness that much and just want to use their favorite legendary pokemon. If you have ever made an in-game team with random movesets that try to hit everything super effectively when you were younger then you can see where these players come from. Other tiers take a bit of learning to see what is usable and are restricted in ways that may not appeal to the players this tier attracts, hence why teams are competitively questionable at the bottom of our ladder.
With 0 EVs and 30 IVs in speed with a Timid nature, Xerneas hits 512 speed at +2. This is enough to outpace the entire unboosted metagame (Deoxys-S hits 504 speed with max investment and a boosting nature). You will be outsped by fast scarfers, specifically those with a base speed of 106 or higher. If you invest 116 EVs into speed (with Timid and 30 IVs), you'll be able to outpace Scarf Darkrai and avoid an untimely Trick/Dark Void.
The choice is up to you really, depends on what you feel like outspeeding
If I may ask, why a White Herb in particular? Do you mean a Power Herb for Geomancy?
Scarf Darkrai is not a relevant speed tier, its just something that is annoying as hell. And yes he obviously meant Power Herb, why do people make that mistake so many times.
My mistake, but if was mentioned before I got here, evidently it's at least worth considering for someone.
I generally carry more than one, especially on Stall. The best way to deal with Geoxern as of now is Klefki who can paralyze it, break its Sub with Fairy Aura Boosted Play Rough, and get off needed damage with said Play Rough. It can also perform other tasks which is an area where a lot of other Geoxern checks, like Aegislash, fall short. It is a Spiker and can priority Toxic Defog Arceus formes while also crippling other fast threats with prankster t-wave.
