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.
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.
In the wild, take them down with Wyvern breaths or leave them alone.
If one of those is employed by enemies, make use of Compound Bows or Velonassaurs in case getting near isn't a possibility.
If you absolutely need to bite those things, use a Reaper King, as they reduce any source of damage - aside poisonous breaths - when not affected by light, including damage reflected by Arthropleuras and Kentrosaurus. Remember, damage reflection ignores saddles.