To get this achievement you must get a headshot on 25 enemies while they are using the HRV.
This is easiest to do with a buddy or two, having them turn their HRV on and letting you do some headshots with your favorite pistol.
Simply invite them to your lobby while in Intermission, and try to either be on separate teams or in a Free for all Deathmatch.
Meet in a secluded place, and farm the HRV headshots.
But say you want this one more legitimately? You don't feel right boosting? I understand 100%. So heres some tips:
This is probably one of the hardest Blacklight: TD Achievements, simply because its hard to sneak up on someone while they are in HRV, and even harder to land a headshot before they can strike back. But it is possible.
First pick a target, one with few, if any teammates around.
Follow them until you see their characters arm move up and touch the visor with two fingers. This is a easy to spot sign that they have entered HRV.
Quickly aim and shoot them as many times in the skull as it takes for the Tango Down message to pop on the bottom left of your screen, assuring you they are dead.
If it was a headshot, shortly after it says "Tango Down", it will also say "Headshot +25xp" (something along those lines). All of this will show up in the bottom left.
Sadly, there is no message that says if they were still in the HRV as they died, but if you act quickly enough you shouldn't have to worry.
But remember, speed IS a very big factor here. The HRV only last for a few moments. During those moments the enemy cannot attack, but can see exactly where you are with very little effort. This is why teammates near the target are a problem, because if your location is known then you are one quick call-out from death. Try to get the shot in before anything like that happens, or before it runs out in general.
I know it may sound a bit difficult, but if you keep trying you will eventually get it. Goodluck, and I hope these tips helped.