This is major difference between the good premades and ok ones.
It's actually pretty simple to kill someone in this game. DPS > HPS = person dead. As most learned in 6v6 scenario the Ironclad, you can't just focus fire someone and kill them if the opposing team is at all decent.
In order to kill someone you need to stop the opposing teams healing and mitigation. For example, silencing a healer gives you 4 seconds to kill someone. Knock backs are best used to negate the effect of guard, separate a player from the tank guarding them.
One thing you will often hear in a good premade is one player silencing one healer while a second silences another. (Knockdown or stagger works here too, but I have a silence as a SM, so I use that). In a typical scenario, you've dropped the effective healing output of the opposite team in half which should give you enough time to drop any DPS. The healers will then try to rez which stops healing for another 3 seconds (4.5s if you disorient the healer). Focused Mind is good for getting rid of the silence to save a wipe. Usually the team that gets the first kill is going to win the engagement.
Brute force does not win at the highest levels of the game. Timing and finesse do.