well best 1st slot depends on how you snipe.
Cloaked so it will be harder for people to find out where your camping.
Fleet foot if you like running around with the sniper and getting into gunfights
scoped-in is great for either
weapon expert if you wanna do the raffica trick
2nd slot, I'm not so sure but I see it as:
bargain to get your t-bolt quicker
ammo award to get 5 bullets for every 3 kills(not really a problem if you die a bit)
monkey man to climb to your camp spot quicker
ammo award should give you ammo for every commando and gunslinger medal you get, which is every 3 kills with that particular gun.
Take note that if that 3rd kill is with a power weapon such as the t-bolt, you will get no ammo. Also it doesn't have to be in the same life for it to activate, I'm just saying it really is only useful if you don't die as you won't respawn with max ammo to often.
scoped-in is difficult to level, the way I leveled it personally is in 3TDM with a buddy. I used Sully's special loadout so I can have for my buddy, and my friend had callout on his weapon so he can point out guys for me and everytime he died I was able to see who killed him. Also since its 3TDM it isn't as hectic as other DM modes and I'm not ruining the match like I would if it were plunder or TO.