[ds5400]

I've thrown the titan harpoon at the earth titan about 500 times but he won't die. It shows his energy level bar about half full, but it won't go down any further.

[foobar]

You have to wait until he opens his mouth to roar. There will be some orange glow in his throat. Then aim for that. You'll know it worked when you see a big orange explosion. If there is only a small greenish explosion, you missed. After maybe a dozen successful hits, you'll get a cutscene with a quick time event to perform the killing blow.