Does akali kill a might 2 unit with master yi as defender legend

Yes — an Empowered Akali kills it.

Master Yi, Wuju Bladesman's +2 ("While a friendly unit defends alone, it gets +2 might") only applies to a unit that holds the Defender designation, which is granted when the Combat Showdown opens — after the Chain empties (Rule 464.2.c.3). Akali's "When I move" trigger resolves first, while the unit is still a plain 2 Might, so her 2 damage is lethal (Rule 143.2.a). Same timing as the Shield defender bonus (FAQ #12332).

Two caveats: if Akali isn't Empowered she only deals 1, which won't kill a full-health 2 Might unit; and once the Showdown actually opens, that surviving unit would get Yi's +2 for combat damage.