The following are suggested ways to say "be relentless" and/or "be indefatigable":

qaplI' insist 

jegh(taH)Qo' refuse to give up

leS(taH)Qo' refuse to rest

Qu'chaj qagh pagh len zero breaks/recesses interrupted their mission

yevbe'lI' un-pausing [telic]

nIDtaHqu' very continuing trying

See the original request on the KLI wish list for more information.