You can try to use the "skill" argument, but having melee DPS makes the encounter harder to complete, especially if your DPS don't notice that he gives you about 1.5 seconds to move away before he actually spits the poison at them.
I finally killed the damn dragon this morning. Took a MNK, WHM and BRD but we managed to do it. No, I don't feel "accomplished" or any of that nonsense, and am simply glad to be able to see the story post-Longstop.
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As a whole Longstop is definitely overtuned and in need of adjustments. I ran a higher risk of dying there than I did fighting the primal that comes after that dungeon (said primal was a fun fight and well-tuned, by the way). The fact the dragon has a stacking poison yet one of the two healers in the game cannot remove it (SCH doesn't get their status removal spell until lv40) is a very bad sign. The run where we won was mostly lucky, as the dragon decided to spit mostly on the healer and the BRD, who kept far away enough that I actually did not run out of room during the fight. God knows what would have happened it if have been PLD (me), DRG, MNK, SCH...