We recently got a Dragoon to replace a player who left our group, we haven't had the chance to take him into T10/T11 yet so I can't comment on them but on Whitefires in T12, we never even noticed a potential problem. The only time it became hard for the Dragoon to survive a Whitefire is when it became hard for everyone to survive a Whitefire, when someone made a Blackfire burst and give the vulnerability debuffs.
I'll see how T10/T11 goes on Tuesday with him.
I'm gonna bold this as people seem to think it's Blood for Blood causing the death, it isn't. It's their party composition (and the Tank LB being late, but their argument still stands, as everyone else lives).
With T11s Nerve Cloud, since the attack is a 100% critical hit, the effect of any lower defense will be amplified (80damage x 1.5 = 120, 100 x 1.5 = 150, from 20, to 30 difference for example), and since you lack a white mage to give the proper version of Protect that will be amplified even further (IIRC Protect is +20% MDef, +20% Mdef DOES NOT equal 20% damage reduced), so I can see where the issue could potentially come from, but as I said in T12 we haven't noticed any issues with him being unable to survive Whitefire hits, even with a vulnerability stack the highest we saw it hit was 5.5k.
Frankly I'm not sure where the blame should be, you guys run an irregular set up with 2 SCH so you lack Magic Protect, with this in mind you also take a DRG which lacks in magical defence to other classes, it should be no big surprise that he's taking a lot more damage than others.
Which moves the argument on to, should Coil be beatable with any 2/2/4 party set up? It's become very standard at this point to take PLD, WAR, WHM, SCH, BRD, MNK, SMN/BLM + 1 other (usually a second monk in hc progression), as it's the most optimal, giving you a good range of utility and mitigation across the board, access too all limit breaks and since atleast 7/8 jobs are different, good speed LB building. I would say yes, it should be possible with any 2/2/4 but any challenges that come up because of it is on your head, for knowingly taking a party which is not optimal, not SE.
That said I am interested to see what the Dragoon buffs end up being. If they buff it's Mdef the Pdef will have to be lowered to be on par with other jobs. Funnily enough BLM/SMN are actually on the opposite end of this spectrum, our PDef SUCKS but our MDef is great, but since raid-wide attacks tend to be magical, people don't notice that much.