Aside from the sheer arrogance of deciding that you get to narrate what happens in other people's games, that's simply not true. The giant is not strong enough to smash a (5.0, obviously) druid wild shaped into an earth elemental into paste with one hit - the 80 damage becomes 40 damage (resistance to nonmagical B/P/S) which is less than a third of the health of just the wild shape. The druid can stand there, take 3 hits from the giant, then shrug off that damage with a bonus action in the next round.
Eh, no. No arrogance or deciding how you do your game. I'm talking about how RAW works. By RAW you aren't even truly hit until 50% hit points, and then it's scratches and bruises until you hit 0 where the solid hit happens. Thems the rules.
If you guys want to play Dragonball Z PCs where boulders thrown by giants are bouncing off of your PCs because you have 200 hit points, go for it.
Without any bruise at all, because rock doesn't have a circulatory system or skin, both of which are required for bruising.
"DESCRIBING THE EFFECTS OF DAMAGE
Dungeon Masters describe hit point loss in different ways. When your current hit point total is half or more of your hit point maximum, you typically show no signs of injury. When you drop below half your hit point maximum,
you show signs of wear, such as cuts and bruises. An attack
that reduces you to 0 hit points strikes you directly, leaving a bleeding injury or other trauma, or it simply knocks you unconscious."
Give me a break. Then some OTHER sign of wear and tear.
Until you reach half hit points, there are no signs of damage on your elemental form. At half hit points down to 1 hit point, it's minor wear and tear WITHOUT a direct hit. At 0 you are hit directly by the crossbow bolt which cracks you in half or something, causing you to shift back to your normal form if playing 5e, or are knocked unconscious/killed if playing 5.5e
P.S. You are also not truly rock in 5.5e. You retain your creature type and do not become an elemental, so you would still have your blood and circulatory system.