I main as pld and sch and generally do DF as paladin with my whm boyfriend. Here are a few things, not in any particular order. If you are going to only try to maintain threat on one mob, mark them so that the DPS know not to go all out with area damage. That is the number one cause of "bad tanks" - when there is a pack of mobs running all over the place because of not enough area enmity generation. If there are 3 mobs and you've not flashed at least 2 or 3 times before the healer tosses out a few cures, some mob is now munching on her face. I've seen tanks loose mobs after ONE regen. No excuse for that.

I don't mark mobs for kill order, I can keep threat on everything doesn't matter who's doing what...flares, holy, single target DPS, it doesn't matter...so I know it's not that difficult. SE has buffed Flash with the last patch, it's veeeerrrrryyyyy much more effective now in early content. Tanks are for holding mobs, don't concern yourself with doing extra DPS.