Struggling with the kill unalerted Tusk Ballistas riven challenge

do it solo. a squadmate running in alerts the whole camp and every ballista in it stops counting. they're the snipers at the Grineer outposts on the Plains, so pick an outpost, stay at range and headshot them with a bow or anything silent. an invisible frame makes it a lot less fiddly. if an alarm goes off, don't keep fighting there, move on to a different outpost.