Encounter design is always some combination of giving players chances to use their characters' moves and feel powerful and heroic (Be a fan of the characters), and putting them in situations where their usual toolbox doesn't work and they have to get creative (Think dangerous).

"That doesn't work quite how you expected" is always a tool available to the DM, so long as it honestly and plausibly follows from the fiction and isn't just something you thought up on the spot because the players cheesed your cool encounter with a move you'd forgotten they had.

Giant: falls asleep, because a giant suddenly falling asleep is never not funny. Killing it in one blow while it's asleep is not guaranteed. Oh, and it made a lot of noise when it fell.

Centipede: centipedes don't sleep, at least not in a way that is comparable to how we sleep.

Hydra: Hydra heads each have their own brain, ergo they sleep independently. The confusion that results from 1d4 of them falling asleep creates an opening, albeit a small one, for some heroics.

Basilisk: did you know that basilisks sleep with their eyes open? Now you do.

Dragon: dragons are primal creatures of ancient power. Do you really think your simple hedge-wizard magic is enough to make one just drop from the sky?