A general rule of thumb that I find works well is to flash once for each mob in the group that you pull, and then cycle through each mob using your Rage of Halone combo until all the mobs are dead. So, for example, say you come across a group of two crabs and a bird: pull the bird with Shield Lob (bird is the most dangerous because it has an aoe attack), flash three times once the mobs are around you, then Rage of Halone combo the bird, then crab 1, then crab 2, then the bird, etc.

If a BLM or WHM is sleeping the mobs, you can change this up a bit by flashing a bit more and restricting your Rage of Halone combos to whichever mob is being worked on. Even there, though, you should probably start your combo on the next mob before the first one is completely dead, to ensure you have a good lead on emity before the DDs set in on it.

Marking mob order is good, but it takes time and depending on who you're with they might lose patience. While ignoring their complaints is an option, if you want to roll with it a good tactic that DDs will usually go along with is to "mark" enemies with Shield Lob. Sensible DDs will attack damaged enemies first before they will attack undamaged enemies - Shield Lob will slightly damage a mob, which makes it a more likely target for the DDs to go after. If you're playing with a ACN, SMN, or SCH, though, this falls apart as their DD strategy works best when they load up all the mobs with as many damage-over-time effects as possible. In those cases, you can "guide" DDs by ensuring that one of the mobs is more damaged than the others by focusing on it while they finish off the previous mob. DDs will usually go after mobs with more damage on them than less.