Good Glenrick

"Good Glenrick" subdungeon is the main reason why it's preferable to search for hidden subdungeons early. Glenrick should be killed for bonus experience as soon as possible. The best case scenario is when you find him when you are still L1, and have IAMAWAL. Then casting  IMAWAL for bonus XP, and then killing Good Glenrick gives you enough XP to become L4 instantly.

In EE mod Good Glenrick receives a significant boost to his stats thanks to the “Death Wish” mutator, but his level remains below 10. Assasin's  SWIFT HAND should quickly take him down