JP wiki
This specifies a non-rage overdrive that's single-target and applies a massive attack buff, and a rage overdrive that targets the entire team and applies a second attack buff that stacks with the first one. With 1 buff the boss's attacks multiply several-fold; the Expert boss's buffed overdrive deals about 3k damage unmitigated. The boss's overdrive meter is 2, so she'll buff herself every 3 turns normally, and every 2 during rage, which will quickly outstrip the cooldown of Cassiopeia's Chaos Magic.
There's no comment on the Ultimate boss, but expect 4-5k instead of 3k.