Quests like this where you have to last-hit someone in order to receive award were in era since the start, same thing happens every time and i dont agree that it has to be changed. First of all, if you dont like mining and getting spiders stolen you can always become one of those who just run around and steal spiders. Secondly, its just like real life, you work hard, you try to achieve goal and then someone steals everything. Finally, heres what i did, there will be always those who see your spider and try to kill it, so i suggest you not complaining about that but if you see another spider in your room, you try to kill it too. Other diggers killing your spider, you kill others spiders so in total you will last-hit just the same amount of spiders as if you would try to kill your own by yourself.