Ethereal Queen in Valkyrie Profile 2 the first time I fought her. It took me over 4 hours. On subsequent plays I've taken her out in a matter of seconds, but she's an optional end game boss, and i was fighting her with main story equipment and stats, and hadn't been doing any Einherjar releasing for stat boosts. Those fights were all designed for players who'd been boosting their stats and who had already gone through and looted a bunch of bonus gear and abilities and shit from the Seraphic gate, whereas i just kinda bulldozed through it (including the other 4 end game bosses, the first of which took me about 35 minutes, and got progressively harder after each boss). The reason it takes so long with those stats is a combination of these bosses just having such ridiculous amounts of health compared to your damage output, and you having to play it extremely safe to make sure you don't get your entire party one shot. Whereas with a normal fight I'd just get everyone healed up and then blitzkrieg a boss with everything i had, doing something like that with these guys was most likely going to result in your entire party getting blown apart by an area fire spell 2 seconds after your attack. So Instead I'd usaully have to split the party, have a bunch of people hiding, and the strongest guy I had trying to flank and spank while keeping the heat off of everyone else.
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