In the 12-level PC version level 6 is a simple corridor with a single fat guard, who employs a fighting style similar to Jaffar's.
Level 3 is effectively split in 2 sublevels: an obstacle course, after which the state is saved, and an immortal Puzzle Boss.
Same with level 12. First sublevel is a long obstacle course with Shadow at the end. Second sublevel is several simple traps and Jaffar.
Even disregarding that many of these are sublevels, even the simple traps of Level 6 and Level 12(bis) are enough to disqualify them as sublevels. The second half of Level 3 also has a bit more to it than the skeleton.
Removed these purported examples of Boss-Only Level:
Even disregarding that many of these are sublevels, even the simple traps of Level 6 and Level 12(bis) are enough to disqualify them as sublevels. The second half of Level 3 also has a bit more to it than the skeleton.