The best way to get Xeric's talisman is by killing regular or brute lizardmen. They have low combat and low defense. They are mostly found in multi combat zones. This means you can use a cannon to kill multiple of them at the same time. If you are lucky, and you have done the Kourend and Kebos Easy Diary and thus the drop rate is doubled, it will only take you about 20 to 30 minutes to get one. You have a one in 125 chance of getting the Xeric's Talisman as a drop here.  If you do not want to do combat, it is going to take a whole lot longer. You will have to thieve the stone chests. These have a one in 300 chance of being the loot in the chest. So you can be here quite a while till you get one. If you have a low thief, do not forget to bring a lockpick.