I’ve tagged this as a bug but it’s really more about the design of the area.
At Ymir’s Delusion it’s possible to kill all the Frost Giants w/o taking any damage. Just get on top of a shelter (there are 4 of these) and shoot a Frost Giant with an arrow to get it to agro on you. Then just hack away with your melee weapon of choice once the Giant glitches into the shelter. I usually use the southern most shelter when testing this to see if it’s been fixed or not. I’ve been hit although rarely when I’ve climbed onto one of the other shelters around the bonfire.
This also leads to possibly exploiting Ymir’s Delusion for experience and loot (provided the Giants are dropping loot as they were not the last time I went there). Once all the Giants are dead, if you leave the area and go a certain distance away then return to Ymir’s Delusion, the Giants will respawn on SP.
The obvious way to correct this glitch/exploit is to remove the shelters.