Ben-Tsiyon Tomer has written:

'Malakh enayim'

The names of the eight musical modes are the Adonai malakh mode, the Dorian mode, the Flamenco mode, the Lydian mode, the Locrian mode, the Mixolydian mode, the Phrygian mode and the Lonian mode.

To get 'Malakh', Yosuke's strongest weapon, first you need to have beaten the game at least once.
On your 'New Game+', a new enemy called 'The Reaper' will randomly appear when you open a normal treasure chest. If you beat him he will randomly drop a weapon. an armor or the 'Omnipotent Orb' (This last one being the harder to get).
I recommend you to be at level 70 or higher since it's one of the hardest enemies to beat and to have Yosuke in your party as well (This will lower the odds of getting it).
The Reaper appears more frequently in the real killer's dungeon and usually on the lower floors of the other dungeons.

Note that on the PSVita version, Persona 4 Golden, the Reaper can even appear during your first playthrough.

The cast of Desyatka - 2013 includes: German Akimov as Criminal 1 Alexei Butin as Actor Lybov Bychkova as Teacher Ilya Chernykh as Lyodya Ilya Fedotov as Police Officer Stepan Gavrilov as Pyr Igor Krasnykh as Bum Vladimir Pashnin as Commissar Olesya Ponomareva as Actress Yelena Rudkovskaya as Mother Dmitry Sinitsyn as Malakh Yevgeny Sobolev as Criminal 2 Timofey Yakovets as Valera Yekaterina Yenikeyeva as Sveta

Firuz has: Performed in "Meliksah" in 1969. Performed in "Baba Koohi" in 1969. Performed in "Eyvallah" in 1971. Performed in "Vahshi-e jangal" in 1971. Performed in "Sharoor" in 1974. Played Yusuf Kaptan in "Lekeli melek" in 1978. Performed in "Faryad-e mojahed" in 1979. Played Mokhtar in "Sarbaz-e eslam" in 1980. Performed in "Malakh-zadegan" in 1984. Performed in "Toghian" in 1985. Performed in "Do nime-ye sib" in 1992. Played Rabiee in "Jahannam-e sabz" in 1995. Played Firuz in "Badalkaran" in 1997.