I get Cas is a high damage character, but I did not think Karn was Mortal Kombat X-rayed. 

Yeah, Cas is a high damage character, but he is mostly utility based. 

I'd classify him as an Assassin type. Simply because... He can hit really hard and knows where it hurts, but can get overwhelmed easily. 
So very synergy dependent. But like everyone, he has some means to defend. 

Poor Karn, she should ask for a shot, she'd at least be healed under Monica's care.