Yes you can, dragon lords servant states it cost on less to cast. This does not change the cmc of the card itself.
June 11, 2015 midnight
Sorry. I misread the comment. Disregard my comment, if read.
June 11, 2015 12:05 a.m. Edited.
Billpasdmf read my answer its the same as yours. Yes he can use disdanful strike cause it does not change CMC. I just explained it in less words then you. Please learn how to read properly before correcting me.