You have to have a powerful siren, level 30 or so, and you also need five points in the blackout skill.

You have to go up to a lare group of badguys and activate phasewalk, but you have to make sure that three of the badguys die instantly.

Your phasewalk should fill up while you are phasewalking.

When that happens, you are able to do melee attacks as much as you want without the phasewalk ending.

In turn, as you do it over and over again, your bar should fill up again!