A Blood Death Knight player has soloed Mythic Sludgefist on live servers.
Celalinda defeated Sludgefist on Mythic Difficulty alone at Item Level 304 as a Blood Death Knight.
It took 1 hour and 50 minutes to defeat the Castle Nathria boss. Sludgefist is one of the few bosses without an enrage. Full logs are available here. Below is a sped-up version of the solo.
Alternatively, you can watch the full VoD.
Source: Reddit