While battling a lone kentrosaurus or even a herd it is highly advised to use ranged weapons. Due to the kentrosaur’s slow movement speed and ability to reflect moderate melee damage using hand held weapons and attacks with smaller dinos can cause serious destruction to you and your tames.
It's best to leave them alone unless you want to tame them. They act like spike walls, so you and your dino take damage when you melee it, and it's a lot of damage, even low level ones do a lot. If you want to kill one, it's best to use ranged attacks like a gun or a wyvern blast.