lol was this the hypergate? because that game the enemy had very little dps. And speaking as the merc healer, I had kolto shell on him which would have been ticking on him regardless of range, He's a very good assassin, your team just had very little dps and couldn't kill our tank... Definitely not hacks, just someone who knows their class well.
I've been called a hacker for not running out of energy on my scoundrel and operative when im keeping up with heals by just using hots and surgical probes because the enemy's output is low. Tried to explain the concept of tactical advantages to the guy but sometimes it's just not worth the bother to explain to people like that.