What are the markings on the rocks near giant camps? And whats an easy way to kill giants?

Distance weapons, my dear, distance weapons.

I make "Slow" poison out of deathbells and salt, it lasts quite a while & gives you a better chance. "Paralyze" poison (canis root & imp stool or swamp fungal pod) doesn't last very long, so you'll need to carry a few if you go that route. "Paralyze" weapons don't always work, whereas poison always works on giants.

Anyway, conjure up an atronach to help, use poison on your weapon, & take him out with arrows. You can also take him down with magic, I like to use fireball or Ice storm.

Hmm never thought of that. Thanks for the tips. But any info on the markings on the rocks?

There just that, marking. Possibly to mark their territory or ward off potential intruders.

Lure dragons to them. While they duke it out against each other, you can attack the giant. Just watch you don't get clubbed to one of the two moons.

As for the giants no where near dragons, level 30 + enchanted weapons + perk boosters + companion to act as decoy. Two hits and boom... down they go.

You asked for easiest, not challenging.