Also keep in mind that not every rat has the same defense level even when the server is running normally. For instance I was in the arena today and new player was having trouble with a rat, no matter how many tmes she hit it with the knife it took no damage. I had a couple of spare short swords so I gave them to her, she killed the rat in a few strokes. The rat had a higher defense level than the player could overcome wth a single knife, another rat elsewhere may not be able to resist, so if a rat is impossible to kill, it may not be you, it may just be a tough rat that needs better skills or weapons to overcome.