They adjusted level sync gear a bit (upward) since I leveled MRD/ WAR, so there might be a bit more strain, but I'll give you the very simple advice that was given me around Halitali or so: Overpower is your friend. Don't run in and use it once. Use it at least twice right away, mark a target (actually if you're marking it's best to do this ahead of time), get maim up at least (or complete a combo if high enough level) and OP some more. If you get low on TP you still have a flash or two that you can pull out.

Ordinarily 2-3 overpowers are sufficient to secure threat on the entire group as long as DDs don't go attacking completely different enemies making you cycle through them. You should be able to secure the whole group and then work each one down.

I should say that I have much more experience (incidentally I suppose) tanking with mostly single target DDs. More aoe from DPS = more aoe from you. Of course once you've got WAR up to 45 Steel Cyclone is another great aoe tool, especially to grab intial threat (paired with Berserk preferably, and/or unchained if possible). I haven't gotten back into tanking since HW (my 3rd combat job, but I also prioritize it behind half a dozen gathering/ crafting jobs) so after 50 I don't know. At 50 though, holding threat become more or less a non-issue/ forgone conclusion. If you're playing at all competently at 50 and someone pulls a mob off of you then they're doing something very special (or they're also a tank).