When completing the rift missions, you are required to get a certain score from collecting data packages on the map, after this number is reached the player can then rescue the trapped assassin, but a new higher goal will also be posted. Most of the time freeing the assassin will give a massive boost so don't worry about running out of time, you should be fine.
Normally the first goal is like 500 and the second goal is 1500, but getting the assassin gets you 750, so you'll only need another 250. Do this 3 times and you're good to go.