Same thing as always: bosses aren't automatically tanks just because they are bosses. Id has horrible HP - 3000 HP against a damage average that hovers on the 600 area effectively means that, at stat topic figures, he gets 5HKOed by average damage in-game (less than two party turns!), and it only gets worse if you take less generous stances than Dhyer's. Not to mention his defense is below average (although it doesn't seem that relevant), which doesn't help at all.
tl;dr, unless you don't scale boss HP at all, Id is horrendously frail, and a harsher boss HP scaling method can reasonably see him OHKOed by Nel with relative ease.
EDIT: Also, just for giggles, I might want to add that, if you don't scale numbers -at all-, ignoring relativity, Nel crushes Id for raw HP and damage, but that's SO3 curves in general being silly and liking four-five digit values, and in this case, Id would be a Light while Nel would be a shatteringly brutal Godlike. >_> Not to mention the idea would be sorta stupid anyway. <_<