>>3837467
You can swap her weakness at any time by using the Special command with someone. Yolnaya Wild Life + Charge + High Fury with a light damage part and Plamia Hearts equipped is the strat, you can also use the barbarian lunch before the fight to increase your crit chance and possibly oneshot her.
Do the Mechagod ruins if you haven't yet as well, it increases the level cap to 150. 99 is plenty enough for Idea, but she's still pretty tough at 99, so going higher will make it easier.
You can bind on turn 1 with Dita, drop defenses to 0 with Grim Torture, then use the special command on turn 3 to swap her weakness to physical just in time for Yolnaya to attack.