I find that the npcs stand on the worse side when the boss goes into "spin-it-to-win-it" mode. During the spin mechanic there is a dot effect if you stand too close to the mob, a huge 1/3 of the arena rear conal attack and a slow traveling series of aoes from the front of the mob. the npcs all like to stand to the mobs left (from the perspective of the boss) as he spins clockwise. I find it much easier to stand to his right. Also, as a blm you won't have much time to cast longer spells like fire 4 during this phase. It can be done, but the timing is tight, if you don't have triple cast up. play conservatively and use faster casting spells so you can continue to move with the safe area.

If you have trouble seeing the aoes, you might try playing in colorblind mode, as well.

ShadowBringers is significantly less forgiving of standing in things than the game has ever been before. As a blm playing with trusts prioritize not getting hit by boss mechanics over finishing your spells... the fights are going to take a long time no matter what and a lot of the mechanics can kill you in one hit OR will give a stacking vulnerability debuff that you can only have one of, before the next thing kills you.