I would have thought the pistols would be the hardest (well, the executioner anyway). didn't have any problems with the snipers, smgs or shotties... just about got the ARs done (none hard there so far) but haven't started really with LMGs so can't comment there.
SMG, LMGs, AR, shotties and snipers are all relatively easy to get gold on. It takes time but not excrutiatingly painful. Only primary I consider a pain is the shield.
secondaries : pistols are a pain. Launchers minor difficulty. FHJ118 was the hardest but not really that bad. Crossbow and ballistic relatively easy because you can do them in sticks and stones. Combat knife painful.
In core, Shotguns and Snipers will take a while just because of the S12/M1216 and the SVU AS.
I think the specials are the most difficult to get, but the hardest weapon I've gotten gold so far was the Executioner. Thankfully, knowledge from that has made getting my other guns to gold so much easier
The easiest primary group to get gold are Snipers, no headshots needed and they are all an any range 1 hit Kill weapon. I breezed through them and I'm average.
The LMGs were quite easy too with the aid of the Target Finder for headshotting head glitchers.
ARs and SMGs are easy mode anyway as the game is made for run n gun SMG users. ARs are very easy as well.
The hardest primary group by a country mile were the shotguns because of the range limitations.
The hardest actual primary I would say is the Riot Shield.
The executioner by far, I do have it gold though (seriously you can shoot someone in the head from 2 meters away and 70% of the time you do't get a headshot )
The shield maybe unless you play hardcore.
The knife can be a pain when you go for the bloodthirsty's because sometimes it just does'nt connect.
The Executioner all I did is run around playing my usual style with PDW and anytime I found a clueless sniper/camper I pulled out my Executioner and headshotted them. Then just go to hardcore for the 150 kills no perks no attachments, people drop like rocks. Should be able to complete most of the double kills, revenge, bloodthirstys as long as you keep to small spaces to do your fighting.
Hardest for me was Assualt Shield by far... but I actually had the best luck in Domination due to the predictability of players and being able to flank them. I was getting completely raped in HC when I tried.
The Bloodthirty on Crossbow was a pain in the ass...finally completed that in a game of sticks and stones after trying all the other tips like FFA with Tri-bolt and scavenger or HC.
The next worst thing was the 10 player controlled scorestreaks with the FHJ. I suggest you all get your basic 100 aircraft out of the way and keep a class with black hat + FHJ + Cold blooded so that you can take out any Lodestars or VTOL warships. 9 of my 10 were Dragonfires and those are few and far between. Or when a Dragonfire finally shows up teammates shoot it down or EMP it instantly... The damn thing takes 2 shots from the FHJ or you have to shoot it with your gun and then lock on with the FHJ. So many times I locked on and was about to shoot and then got killed... it's maddening.
I play only hardcore, and for me the AR's and SMG's were easy, sans the SMR lol. The pistols haven't been bad so far, but the executioner is tough to get the headshots with, the 5-7 and B23R are the easiest. Shotty's seem to be the most difficult on HC because its so hard to get bloodthirsty's especially with the pump action shotguns
My first gold gun was the Vektor K10 SMG. The bloodthirsty medals were really hard to get because I have poor reflexes and am a 0.5 K/D player. Also, in BLOPS2, there are so many players using SMGs and shotguns and it makes close quarters combat hard to stay alive.
I now have two of my LMGs gold and it is much easier because I can hang back and camp. The drawback is that getting headshots is time consuming with LMGs because they are slow as molasses.
However, the bloodthirsty medals are much easier with LMGs than SMGs.
Next I'll try snipers in hardcore mode because every kill in HC is guaranteed to be a one-shot-one-kill medal and I'll be able to get those 250 kills easily, much faster than headshots.