|Kill or capture all HVT. 5 are story related. 50 are side missions, just look on the map for HVT targets. According to PLmikez (post here), two can get blocked off if you beat the game before you kill/capture them. (If you take china's path.)|
You will also be missing one person for China or AN, depending on what path you take. There is a total of 56 HVts in the game but you can only kill 55 of them. Thus why its 55/55 in the progress menu. You do not have to kill the one you are missing. You will still unlock it at 55/55.
Not only that, after beating the game the game, China or the AN (depending on what mission you chose) will not be able to have any progress towards the HVTs, If you did not talk to that faction them before hand. Just don't do the last mission before doing a mission for both, and it should be fine. If the HVTs read 53/55, you will have to do the last mission for the last two HVTs. (Go in then if you are not going towards Digital man or already did everything else.)
Try not to kill them, as there is another achievement (Aces High) for capturing them all. Theres a nice Post over at the Pandemic forums about the HVTs here.
Warning: If you join someones game and kill all the hvts, you will not unlock the achievement. Another Warning, if you beat the game, you may not be able to do missions for the other faction. So be sure they are all completed before hand.