After some investigation, it seems **Repeater only works on successful shots but if you have Stock then that damage can trigger Repeater bonus and execute the enemy**. 

![animated GIF showing what's described above][1]

Animated GIF source, and more info at: https://www.reddit.com/r/Xcom/comments/44oxea/stock_repeater_flawless_execution/


  [1]: https://i.sstatic.net/FV1Hk.gif