With regards to the 20 Unica headshots, I absolutely farmed a shedload of Unica kills while on 1PARA's server before. Operation Locker, chokepoint between C and prison cells (D?), attacking towards D. Kept it aimed at roughly head level and blasted away. Pop out, aim at nearest crowd of unwilling cannon fodder, hammer Mouse 1, back into cover, reload, repeat.
Helps if you're Support class as well if you're getting kills at a chokepoint, gotta lay down dat ammo box as you WILL use it up quickly.
Even if you don't get all of your headshots, you should be well onto your way of unlocking an RDS. These help massively for more accurate shots, rather than iron-sighting willy nilly into a crowd of suckers.