Hopefully no-one is as stupid as me, but just in case....
I read that as soon as you unlock the first mission, you can just press X at the end of the mission to go onto the next one, so upon completing the very first mission I just kept pressing X until the achievement unlocked. It took me ages to realise that I was just playing the same two missions over and over again....
News flash - when you unlock your first Rift mission, you don't unlock all 7, only the first 2! So if you keep pressing X, you'll alternate between the 2 until the end of time. So don't do that (which is what I did)!
The first 2 are unlocked upon completing Sequence 3 Memory 2 (at this stage if you spam X you'll repeat these 2).
The next 3 are unlocked upon completing Sequence 7 Memory 3.
The last 2 are unlocked upon completing Sequence 11 Memory 2.
(See vak 77's solution for the locations of all the Rift missions)
Making 7 in total. Which means you will need to repeat some if you're going for the 'Thawed' achievement (free 10 assassins). Luckily I've done that in advance...!
Hopefully some people find that helpful!