I just had a chance to glance at the Darhold tactics "suggested" in the Eorzea Tsuushin magazine that came out yesterday. To get all 5 chests, the "Eorzea Tactics Squad" strongly advises to bring 5/8 mages, an archer and two Gladiators.

P.S.: They also mention that if one of the Gladiators is able to change to Conjurer after a certain point, things become easier. Isn't that great. Just bring 6 mages and you're golden.

P.S.II: Ah. And all players have to have the archer ability "Stride" equipped to reach their target in time. So don't forget to level your archer. (Of course the latter is voluntary. You don't have to level archer. Only if you want to get the final chest.)