Sure, but it's a bad idea.

You can level a monster down by HD, just like you advance it. I've done it and it works okay. The problem is that many of the monster abilities turn it into a glass cannon: its abilities are much more likely to kill the PCs, even though HP drop enough that they can kill it quickly too.

(Special mention goes to a de-leveled purple worm — Swallow Whole stays very nasty at lower levels.)

Sure, but it's a bad idea

...if taken too far.

You can level a monster down by HD, just like you advance it. I've done it and it works okay. The problem is that many of the monster abilities turn it into a glass cannon: its abilities are much more likely to kill the PCs, even though HP drop enough that they can kill it quickly too.

(Special mention goes to a de-leveled purple worm — Swallow Whole stays very nasty at lower levels.)

If you have an ideal "boss" in mind, and it is more than one or two CR too high, instead either try building a new monster along similar lines at the correct CR or re-fluffing a correct-CR monster to match your ideal (re-fluffing is a versatile and good solution to many problems).