Yeah, so how can you kill him if you do not have 2 knockbacks?
There are some classes with no knockback (Vanguard, Both Smuggler).
So basically if you have a setup of vanguard + scoundrel + gunslinger in any of their specs., you cannot kill the boss.
Gunslinger actually has one knockback by default and another if they took the talent for aimed shot.
They can do both knockbacks themselves.