Seph ex isn't really a hard fight for blm, its just learning where to stand.
Take for instance the beginning of P3, there is actually time get 1 fire cast off before you need to move for twister. So you just do your F1 opener here, F3P to get into position of orb and burst away. The only downside is 50/50 chance your going to be under the right orb, so theres a chance leylines isn't going to be fully utilised.
The only part where the movement causes an issue is the knockback stage of P3 but every job has an issue here.
With each kill I do, im noticing things i can do differently to increase dps. My first run 1000 dps, then 1050. Im up to consistent 1250dps now after 8 kills, with a few more kills i should be able to start hitting 1400+ quite easily.