I did this last night with a silenced rubico prime and a HUGE enemy radar (enemy sense, animal instinct and vigilante pursuit) and lots of patience. (well... i used loki when i got a little impatient and wanted to run to a new hiding spot to snipe from). you probably don't need as big a radar as i had but i had just finished the unalerted tusk balista riven so didn't bother changing set-up. I remember doing the riven OP mentioned and failing as soon as a triangle turned solid red.. i think i got away with one during last night's NW mission early on but it died just as he made his grineer surprise shout. I ran it on Adaro, Sedna since I just seem to have better luck being sneaky around Grineer.