Kel'thuzad - Still never destroyed his phylactery, even @ lv70
C'Thun (since Kel'thuzad was redacted) - actually killed by Med'an in the comics
Kil'jaeden - lolnope! Back through the portal he goes. And even if you don't count him, Akama and Maiev took the credit for Illidan.
Lich King - KS'd by Tirion and Shade of Terenas
Deathwing - KS'd by Thrall and Alexstrasza
So yeah, even if Garrosh dies, don't expect the raiders to be the ones who kill him. Kill credit will probably go to Vol'jin or Varian depending on which faction is raiding, and Blizzard will decide which version is canon later (likely Varian, since it seems Alliance characters are always present at the defeat of the end boss).