When a Pokémon with Beast Boost directly causes another Pokémon (including allies) to faint by using a damaging move, its highest stat (other than HP) is increased by one stage for each Pokémon knocked out by that move execution. If this stat is already at +6 stages, Beast Boost will not activate (not even in the case of a tie of two or more stats).
If the Pokémon fainting ends the battle, Beast Boost will not activate. Beast Boost activates after the Pokémon gains Attack from Fell Stinger. If a Pokémon with Beast Boost defeats a Pokémon with Mummy using a contact move, the Pokémon's Ability will become Mummy before Beast Boost has the opportunity to activate.