Very simple;
When hakkar is about to do his life drain (with about 2 seconds left on the timer) and you've already got your poison from the son, hold down your right mouse button and start spamming the keys ADADADADADAD. Your character should be strafing back and forth as fast as possible. When he does his life drain you won't get stunned and can continue dpsing.
I'm not sure but i think doing this cancels your poison buff on him so it might not be wise for lower end guilds to do this as the dps they'll be substituting instead of the poison damage wont equate. If you can do more damage that the poison itself then this will help you to kill hakkar a lot faster than usual.
Even if you don't want to do it for dps, it is wise to have tank do this so he can keep building aggro on hakkar.