Yanmega is way too strong in the current metagame in RU, my best way to deal with it is sending in uxie on a safe poke and T-wave and hope yanmega switches in on my uxie. Also Jolly entei outspeeds timid yanmega who can threaten to 0hko with flareblitz, though most players seem to run adamant nature over jolly :/
People run Adamant over Jolly on Flare Blitz Entei because Flare Blitz is an event-only move that comes with the 13th movie shiny beast, and it always comes Adamant, just like the event Suicune is always Relaxed and the event Raikou is always Rash.