Because the backlash cap is 40% (for players and mobs such as this one), typically runs (without using scald-incindiate strategies, since those take ~4-5 turns) will necessarily last for 12-16 turns (shad spells last for 3 turns before backlash, 30% each = 4 cycles, 40% each = 3 cycles of 3 turns each, 12-16 turns in total), which I kind of feel like is way too long for most bosses since apart from occasionally performing an action that increases the backlash back up to 40, the only thing you can really do is turtle until the backlash goes off, then turtle again or do another action that increases the backlash before turtling again.
edit: Without the backlash cap being a problem, the fight would be the players and the boss struggling the increase/decrease the backlash amount every turn rather than just have 1 person increase it occasionally back up to 30-40% while everyone else turtles or tries to take out a minion that stuck around. It would keep people's attention to where it seems it's supposed to be: the backlash meter.