Off-hand, I know that this is a requirement of Lola's Sidequests if you want to recruit her on the Goddess Route. The last sidequest she will send you will be to face off against the Imperial Gold Dragon in the Lusamundo Ice Caves (if I remember correctly). If this is your first time, then there is only one suggestion prior to this point of the game.
* Make sure you are at least Lv.35 or higher before completing Rudohke Blast Furnace to recruit your third party member or else you have to wait and do this route again.
Remove all members down to just having males (excluding Pippin) in your active party and go pursue this sidequest boss. The only attack worth watching out for is called Prismatic Blast, but there is no charge notification to when it will use it. For that reason, I would suggest taking the fight on when you're in your 80s or higher on Amateur difficulty. Three combatants minimum really does make it go a lot slower than if you had a full party, but for the Vile God Route, this side-quest does not appear. I do not know currently if you have access to this boss on the third route, so resign yourself to taking it down as soon as you can (which will be the Goddess Route, hopefully).