When Fleshpulper Giant enters the battlefield, you may destroy target creature with toughness 2 or less.
If the target creature’s toughness is greater than 2 when the ability tries to resolve, the ability won’t resolve and the creature won’t be destroyed.
If there are no legal targets available, the ability is simply removed from the stack. If you control the only creature with toughness 2 or less, you must choose it as the target, though you can choose to not destroy it.