Tabbikaat,
here is my strategy of "Leprechaun-killing"
(I beleive each of us has own strategy):
As leprechaun croses the area from one side to another and then returns we usually stand on his road and, when he appears, all of party members that have appropriate spells begin to cast. Spells that we are using are Lightning, Incinerate, Lifesteal - actually all of Ligtning-like spells that inflict multiply damage after one targeting. If we can't kill him at once we just stand there and wait for his returning. Usually leprechaun stops for a while when under these spells - this gives time for completing the murder
Hope this can help, good luck!
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Tanda,
with best regards from Guild of Assassins