Although their claws are sharp, the hide of this Edmontosaurus is really tough and hard to penetrate, so they target a thinner section, and that is the neck.
If you can cut through the thick hide and expose the guts, that is what is going to debilitate that Tenontosaurus and take him out of the battle before it even begins.
" Quick, Tantor" ! shouted the ape-man, and the beast swung him to his head. " Hurry" ! and the mighty pachyderm lumbered off through the jungle, guided by kicking of naked heels against the sides of his head.