I found this little beast und try some spells like silence and reveal but nothing works. i give him some incinerates but the little boy is very tough.

YOu cannot catch him. You can kill him. You don't need to do either. There is a quest from the thieves guild in Ishad N'Ha that asks you to prove leprechauns exists. Later in the game you will find the leprechauns treasure which is all the proof you will need.

Wyv