Every time Irenicus spoke, I just wanted him to keep talking.
I have no idea what level >12 magic looks like in 5e and why it gets so challenging, other than what little I know of Wish, which is in BG2, but magic was a menace in the under level 12 area of BG1 and 2 also. Just frequent spells that would AoE stun your entire party for the next 10 rounds, which may as well have been an instant kill.