The real problem was I had flown my Vendetta off the table when I was tabled, technically I still had some lascannon shots coming their way
I did take off 4 HP from one knight before it ended it the 3rd turn....
Don't get me wrong, a vendetta or tri-las predator, or similarly anti armour oriented unit could take down a super heavy in a single round. You just have to be pretty lucky with those roles; since you need a minimum of two pens with explosion results to off a knight. (Other super heavies, like the baneblade, need three explosions in order for you to have a chance at taking one out in a single round.)