I agree with the OP in that Seiryu is really annoying for melee positionals, not that positionals are hard, but that the fight itself is designed to be very anti-melee with the in-out mechanics, Forbidden Arts, big chungus slams and Serpent Ascending + Crossword aoe. Making it impossible to hit all positionals even with a very coordinated team.

But there ain't many fights that are as bias as Seiryu but I guess this is what they came up with to counter the caster unfriendliness from Suzaku.

Also there are some classes that gets the best of all worlds like Bard (maybe MCH? I don't play it), RDM and SMN. They get all the benefits of being ranged while having minor inconveniences of having to move (due to Dual cast and instant cast spells/dots).