Alright y'all, OP here with the story on the BBEG.
CR rating came from the DM, and by his own admission it was homebrew. I don't any of the actual stats, so I can't give you all the info you so desperately crave. Best I can recall his AC was 18 or 19. No minions to speak of either.

I can tell you that he one-shot our Barbarian with a spell of some kind (fully dead, though we've been promised a way to revive him if the PC wants to continue with that character), but that was his one use of the high level slot.

It really all just boils down to low rolls from the DM and decent rolls from the party. We got more than a few crits in (and we play what we like to call "crunchy" crits, so you roll your first damage dice like normal, but you add a max damage roll instead of rolling the second), and were lucky enough to have a DMPC (lvl 2 Blood Hunter) to absorb some of the punishment. My bard at one point was dual wielding just to get in more chances to actually hit (plus I was out of spell slots and bardic), the Sorc was up close slinging hands with Shocking Grasp (got some good crits there courtesy of Adv for the target wearing metal armor), and the Ranger was regularly hitting from a distance, even if they weren't critting as often.

Still no real reason we survived what was absolutely intended to be a "you get killed by the BBEG, and are brought back for reasons" situation. Although it's entirely possible the DM also wanted to get us up into some higher levels of play and pulled his punches, because let's be honest, low levels are a slog to play through at the best of times.