Always start a pull with Shield Lob. Always. If you do, the DPS are likely to follow your lead and begin attacking the target that was hit with Shield Lob first so that you don't have to waste time guessing which target they will hit first.

After that, use flash 2 or 3 times (only do 2 if you feel an urgent need to get threat on a target right away with a threat combo) and then do threat combo on the current target. When you feel that your lead in threat on that enemy is sufficient (as in, you have enough of a lead on a target that you can tell that it will die before it switches it's focus to a party member), switch to another enemy and do your threat combo on it. Try and throw in some flashes every now and then for those stragglers you haven't been able to hit with a threat combo yet.

And at that point all that's left to it is maintaining your lead.