In order to unlock "Brain Surgeon" you must kill 50 players with headshots in competetive multiplayer. This can be done in many different ways, but I'd like to share with you what I did to unlock this cheevo. I'll start off first with my character build.
Using "Cloak" as my main Ego power, I had at the time 5 perk slots to use and I had them maxed out with 3 Ego points in each:
Infiltraitor...Increases cloak duration (+6 seconds)
Shadow's Core...Reduces recharge time of Cloak (-9 seconds)
Shadowed Strike...You deal more damage while Cloaked (+30%)
Ambush...You deal more critical damage to players not engaged in combat (+9% critical multiplier)
Hunter's Stance...You deal more critical damage to players while crouched (+9% critical multiplier)
The core of this build is "Infliltraitor" and "Shadow's Core" to maximize your "Cloak" power. If you are using a Bolt-action Sniper Rifle the other 3 may be considered overkill as this weapon, in most cases, will be a one-shot kill to the skull (you will see a skull being decimated by a bullet in the kill-feed). If you are using a different weapon, this build will give you an advantage on any shots to the head.
My primary weapon of choice was a VBI Bolt-action Sniper Rifle (1271 damage and 3.0 critical multiplier) with an incendiary nano-effect. I would have liked to have found one with a biological nano-effect because if you dont hit them in the head, the nano-effect would slow them down making your next shot easier. My weapon caught them on fire and made them roll around making my next shot harder, but it was still managable. My weapon had a "green" rarity and had 4 mod slots which I filled as follows:
Sniper Recoil Stock...Improves bloom and recoil
Tachyon Laced Extender...Improves critical multiplier
Extended Sniper Magazine...Improves magazine size
Telescopic Sight...Improves accuracy
My secondary weapon of choice was the Grind Fragger. It is Pump Shotgun that has a large magazine capacity and actually fires automatic. In my opinion it is the best CQB weapon in the game and has saved my hide on countless ocassions in multiplayer. I believe I got this this as a reward from a Main Mission so when you have it, dont get rid of it. Save it for multiplayer if for no other reason. Watch your mini-map and switch to it whenever an enemy gets too close and unleash hell!
Now for my preference of game-mode and map. I tried Shadow-War by watching over capture points and found this extremely frustrating. Most players are driving vehicles trying to run players over or when they're on foot capping a point, they're cloaked alot of times making it almost impossible in both cases to get a headshot. I gave up on this and tried Team Deathmatch. Observatory wasn't my style because I found it to have too many hills and not enough clear sightlines, also most of the battles take place in and around the main buildings which make sniping tough. My favorite map hands down is Waterfront. This map is tailor made for sniping. Its almost completely flat, but also has countless elevated positions snipe from. The best part is its usually chock-full of opposing snipers just begging to be shot in the face! Team Deathmatch now has a win count of 25 so matches are fast and furious but I still managed an average of 1 headshot per match usually within the first minute of gametime before most Cloak powers were active. There are a couple spots that I found profitable from each of the 2 initial spawn points that I will detail now.
The first spawn point I will call "A". This point has a tower about 100 meters to your left of where you start. You can run forward and left up a small ramp and then to the tower. Once at the top this place gives a clear sightline to the opposite tower and many other places on the map including the roof of the main building in the middle. The other choice is to run forward from spawn point "A" about 75 meters to a high ramp to your left and run up to the top. From here you have a nice sightline to the doorway of the main building and to your right you can see the boat where snipers like to perch. You also have an area to the right below your position where alot of runners will be trying to flank your team.
The second spawn point "B" is basically opposite of "A". This place has a boat about 50 meters to your left which you can jump to and watch over the high ramp I spoke of at spawn "A". Also you can watch the main building and the surrounding area. You can also run forward a short distance from this spawn and jump up to some barrels or crates and watch the inside of the main building and the opposite doorway which gets crowded quickly. You may want some higher elevation so run forward from spawn "B" and to the right to climb the tower on your side of the map. From here you can see the opposite tower and the ground below watching out for sneaky flankers.
Once you start your run for 50 headshots I have a few more tips to keep in mind. When you are searching for targets with a Sniper Rifle, don't aim! Move your unaimed reticle around the map until you see a sheild/health bar of an opposing player, then aim down sights to locate them. Watch your mini-map for anyone sneaking up behind you and switch to your CQB weapon of choice. Don't just use your map, turn around check your surrounding area from time to time just to be safe. Try not to get frustrated if you miss alot because its going to happen. My 50 didn't come from 50 shots by any means, more like 500. Have FUN and play the game!
Well, I hope this solution helps some players who are having trouble. Like I said in the beginning this is only what I personally did, but there are many other options, so find what works for you and run with it! If this did help you out I'd love to hear from you in the comments and if you hated it, I'd love to hear from you too. Now go cap some skulls!!!
Good Luck and "Happy Cheevo Hunting"!!!