We have a guide on how to use the HMV-II to two shot a tank in Battlefield 4. As always, the video is right here, and a summary is provided below.
The HMV-II is a guided lock-on rocket, just like the SRAW. However, it fires three rockets all at once, producing high damage. So, very light vehicles like drones and one person watercraft are one shot kills, with three rockets, while lightly armored vehicles like jeeps and helicopters are two shot kills, using six rockets. Heavily armored vehicles, such as tanks, would take three to four shots to kill.
The way the HMV-II works is you lock-on to the target, fire, and maintain the lock-on until the rockets hit the target to guarantee a hit. For targets that need two shots, you do the whole lock-on, fire, and lock-on process twice.
Now, you can actually cut down these steps if you’re hitting a target twice in a row. Lock-on, fire, release the lock-on to prepare the second shot, fire again, and maintain lock-on then. What should happen is your initial volley will hit the target as intended, and if the target moves as a result, it will be tracked down by the second shot. Of course, this may not work on flying targets since they move too fast, but it should be OK on land and some water targets. You can do this so quickly that it is almost an instant kill.
Here’s a trick to attack an opponent without giving away your position. Find something to lock-on to other than the target and fire. When that goes off, you can then lock-on to the actual target and fire at it. The initial shot will deflect attention so your opponents can’t tell where you’re coming from.
Now, the trick to two-shotting the tank is picking the right position. Assuming you get lucky, you can catch a tank from the rear unawares, at a 90 degree angle, and you can instakill it right then and there. Remember to use the deflecting attention trick first to distract the tank, so that you can get those consecutive hits in and instakill without any fuss.