So, just to be clear, are you saying that when using Surge you are able to kill the mech while it's facing away from you but the game doesn't count it? Or are you saying that you are often unable to complete the kill while it's facing away?

To be fair I do have trouble completing the kill from behind. But when I am able to line things up so I'm definitely killing them while I'm behind them with plenty of time left on the stun I've employed and in the same position I use when getting from-behind kills via close combat weapons, it's far more often the case that I don't get the kill from behind and I have no idea why.

I don't know, could be a thing. I've all but given up trying to complete the 10 kills from behind with assault weapons. It is strange, I'll get 8 kills fairly easy, then spend 5 hrs on the last 2. I swear sometimes I most certainly got a back kill, but it didn't call it that. I can't compare to clisecquarters because I have seeker revoker so most of those are definitely back kills.

Gotcha. When using Surge I am able to finish this challenge with enough time to spare, so never bothered to notice if behind kills sometimes don't register. I'll try some experiments this week and let you guys know what I observe.

By the way, one non-Surge way to get behind kills with assault is to camp near a control point owned by your team and wait for bots to try and capture it. Bots often run in circles while capturing points so every now and then you'll get a kill while they're facing away from you. Takes longer because of its unreliable nature but works. Overdriver works best among the options I have.

Ngl nowadays trying to get kills from behind with assault weapons (I use carbines since it's the only legendary assault weapon series I have) is quite hard, especially if you don't have the right mech for it. I use either my level 5 rank 6 surge with carbine 8 and 12 loadout, or my maxed out tailsting for more firepower (dual carbine 12) and its signal jammer ability.

Zephyr is a good alternative since it is an epic mech that retains the same emp effect, and the base price along with upgrades are much cheaper now simply because it is an epic mech.

I finally got round to trying this a few times today, using surge and overdriver. In one instance I thought I had one but ended with three :-). The extras were probably long-distance lucky shots. And in another it was one for one. I was too focused on getting control points though so this test is still incomplete. 

Okay so I played a few more games, and with surge and overdriver I didn't notice any obvious successful back kills that the game didn't also notice (in other words, it worked fine). A few important tips / points:

1. Weaken the enemy from a distance first; the surge stun duration is not enough for high health enemies.

2. If you stun a weakened enemy from the front, and if it has enough health, start firing as you're going around so that you only need a few shots by the time you get to the back (and with overdriver it also makes the last few shots more powerful).

3. I use a legendary duration implant, currently at 4 stars and it works fine so an epic one will also do.

so you either have not tried enough times or overdriver doesnt have a bug.. will try overdrivers next time im doing assault from behind