Killed Sinjid's Shadow with a level 25 Sonny (Assassin), level 25 Veradux, level 16 Amber.
Used Break and 1 Shatter Bolt to start, with the extra turn during the bolt I used subversion on Veradux, then block on myself to have a decent chance at avoiding his crazy damage attack.
Break and another Shatter Bolt and his cronies were mostly dead from Veradux and Amber, at which point you just need to focus on keeping him stunned most of the time and using block and subversion to usually block his crazy attack when he gets his mana back. Weave in attacks when you're not stunning, a lot of the damage will be done by your allies though.