Something: A spell, a book, a potion, a weapon, an artifact, a person, a trinket, the holy grail, an Orc Boss or any other evildoer, or one of the "good ones"
1. Go find something and 1a destroy it 1b rescue it 1c activate it 1d trap and bury it 1e use it 1f clean it
2. Bring something savely inside then goto 1a-1f
3. Go to a place and 1a-1f
Never show your players this system. Never use generic objective markers or generic names. It destroys fun if is not Skulmar, but "that random pre boss from the list page three".
It works best when you combine two or more of them: find the spirit blade and kill the witch lord. Find the cultist circle symbols and occupy them at the same time.
Probably it is a good idea to add one or two or three mini-branches in the labyrinth for those mini tasks. Ideally with a creative group separating trap on the way.

Also a good one is to have a pre-boss before they see the real boss, to drain some potions and spells. That one can wonderfully be incorporated guarding the something. Or stealing the something, running away with it; or bringing it into an unusable state. That pesky guy who switches the portcullis lever back each time the heroes turn their back on him.