Christopher asked: "Why "J. a descendu le géant." can't mean "J. climbed down on the giant" ?"

Bonjour Christopher !

The form "avoir descendu" can only mean "to go/come/climb down" SOMETHING: this meaning doesn't apply to people or animated beings.
With animated beings, "avoir descendu" means "to take down -> to shoot them down, to kill them"
If you wanted to say "He climbed down ON the giant", you would use "être descendu" as it would be followed by a preposition such as "de" (off/from) or "le long de" (along):
"Il est descendu du géant. / Il est descendu le long de la jambe du géant. (along the giant's leg)"

I hope that's helpful!
À bientôt !