If you want to kill a diplo because you need prime meat then just place a wooden or metal spike wall. Then lure it to the spike walls and just make it touch the spike walls. It cannot damage the spike walls because they cannot attack but the spike walls will do the job. It will be busy fighting with the spike wall which is of no use. It will start running when it's low. Then just chase and kill it.

Hope you enjoyed my tip. It was useful to me when I was a beginner.