I accidentally killed the guy guarding it because he fell off the cliff next to me, he was tough as ***** until then

Fought him for like 15 times, needed 1 more hit to kill him, and we both jumped off that god damn cliff. Gave me a scrap iron and something else(likely a random candy) and respawned after rest. It's pretty hard to bait him down the cliff(or to just kill him early on), but you can just run past him, open the door, grab the tool then run.

when you aggro him drag him next to the cliff, then jump off into the water. aggro will break and after a lil bit of him walking back, you can just backstab him.

I met him fairly early game, he was pretty tough... I had to use Oil and roast him with the Flame Vent to get rid of him, he was basically filling my posture bar in 3 hits.

So, found an interesting method to finish off the bothersome ninja. Might work for you, or it might not. Right of the small temple where the Mist Raven is located there is a small ledge. With bit of cat and mouse with the guarding ninja, you may be able to confuse the AI to get stuck to higher ground while you remain below the ledge. Ninja, for whatever reason, refuses to jump down, and instead will try to combo you just bit out of his reach. With bit of luck, you may be able to chip away his HP this way.

The trick is to use loaded axe and just spam him when he does the kick attack

go around him and open the door before he catches you.. didnt even fought him

You thinking what I’m thinking? Aim for the bushes? “There goes my hero. Watch him as he goes.” This jumping off the cliff method certainly works. You aim for the water, he aims for the bushes, and dies.

I snuck up on the ninja guard and assassinated him. Idk why it was so easy for me but that's how I managed to kill him after barely escaping the initial fight with my life.