Battlefield 3

This achievement will require you to obtain all of the parts for the XBOW. This is fairly easy, all you have to do is complete a couple of assignments, which are listed below. There is 4 assignments you need to complete (5 including the one to actually get the XBOW). To unlock it you need to get the scoped version, as well as the 3 different bolt types for the crossbow. After you complete all the assignments regarding the XBOW unlocks the achievement should pop
You can do the unlocks in this order:


Assignments

Win 3 rounds of scavenger - Fairly easy, just get it from casual playing, be on the winning team at the end of a game of scavenger 3 times.
--> Unlocks - Crossbow


Get 20 Spot assists - Fairly easy, this can be done with any gun, all you need to do is aim at a target and press select for them to be spotted and then someone kill them or place down a TUG-S and if the person is scanned and killed by your teammate that will also count.
Designate 10 vehicles - Here you need to laser designate 10 vehicles using the SOFLAM, it isnt that difficult just play a match of Conquest on a vehicle heavy map and get scanning. This should be fairly easy if you have Armored Kill and just hop into Tank Superiority and get laser designating. The ones in vehicles also count for this trophy.
Get 1 designated assist - From all that laser designating you should get one of your teammate will kill one of the targets you designated. If not, keep trying and it shouldn't take too long.

--> Unlocks - Scan Bolt (A bolt which will scan nearby enemies and display them on the radar once shot.

Get 50 kills with sniper rifles - This should come naturally from using sniper rifles, and it shouldn't take too long playing conquest or Deathmatch, seeing as other assignments from other DLCs also need this, you're familiar with the difficulty.
Get 10 headshots kills using a sidearm - These are your secondary pistols, and these shouldnt take too long either, but will be a bit tough from a distance. I suggest maps like Norshahr Canals deathmatch or Close Quarters maps where everything is compacted and you can have more of a chance getting a kill with the sidearm.

--> Unlocks - BA Bolt (A long range bolt, which is much better at great distances)


Get 5 kills with hand grenades - Very easy, you'll most likely get it while trying for A Pocket Full of Death Trophy, and it is very easily done on Close Quarters or Norshahr Canals map. Also try and chuck them where the objective flags are on Conquest maps, especially the ones being taken as you're likely to get a grenade kill that way.
Get 5 kills with underslung grenades - The Underslung grenades are your grenade launchers, known as the M320 attachments. Any of them should suffice in getting the assignment done. Again try a close quarter map where if you're at range they have no chance to dodge the grenade.
Get 5 kills with C4 - Get 5 kills with the C4, very easy as you can just run up to vehicles and plant them on, in modes such as Conquest this is very easy as usually the tank is near a base and sneaking up to it and planting the C4 is easy enough. Just 5 kills should be fairly quick

--> Unlocks: HE bolt (explosive bolt which explodes after impact on a vehicle, can be used to also to blow up people)

Total of 150m headshots with Sniper Rifles
Total of 150m headshots with Assault Rifles
Total of 150m headshots with Carbines

All of these are cumulative and are very easy to get. This means if you get fifteen 10m headshots with a sniper rifle you will get that task done. Just keep using each weapon and aim for the head to get this. The ones with Sniper rifles can be done in a couple of kills but the others shouldn't take long either. It's just about using the weapons all the time and aiming for the head.


--> Unlocks - Crossbow Rifle Scope (a scope so now sniping with the XBOW is much easier)


source: ps3trophies.com