Booster 2: kickback endurance or monkey man
One for faster traversal obviously and the other for enhancing your kicbacks longer. I would suggest using kickback endurance by default for the smaller and/or flatter maps. If you get a vertical map, stick with monkey man instead.
Kickback: Speedy G or Gotcha!
Speedy G for catching up to people and snapping their necks with ease. Super great for closing the distance on multiple people, and if you're good you can get multiple stealth kills in 1 kickback. Gotcha! should be used for smaller maps as there will be less places for people to be and you can scope out the stragglers and take them out. Since both of these are timed kickbacks, having kickback endurance can be great to prolong their use.
The game modes I would avoid trying to level this up on are Team Objective, Free For All, Elimination, and 3 Team Deathmatch. These modes can end up being super short and having you lose out on an assassin medal. Team Objective just doesn't offer much oppourtunity as most people fight in close-quarters there so attacking stealthily may prove to be difficult. This leaves leveling primarily in TDM, Classic, and Plunder, plunder being the best choice for obvious reasons.
Weapons: any long gun you are comfortable with, micro or para
The long gun will just be used for getting out of sticky situations. and the pistol will be your aid in getting stealth kills, the raffica and tau won't work well since the tau has spotty blindfire accuracy and the raffica takes to long to injure somebody. But with 5 quick micro shots or 3 para shots, you can injure somebody halfway and help deal a stealth kill. Doing this is primarily to ensure that you get the stealth kill and to avoid smacking the guy in the back of the head and missing out on the kill and possibly dying yourself.
Makes a little difference and getting the randomly chosen map and 1 of your 2 characters to match the map environment can be tricky but having your character match the maps colour can actually help in blending in with the background and add to your stealthy gameplay. Don't overlook trying it if you get some good stealth maps.
all maps are capable of getting stealth kills on, but some are better than others. Try avoiding maps with lost of bumpy rocks or stairs as you have to be near the same height as your prey to get a stealth kill, so avoiding maps like highrise is idea as 1/3 of that map is difficult to even get stealth kills. I find sniping maps to be ideal for stealth kills, and my personal favorite is chataeu. If you notice a sniper in a map, then focus on him for as long as he stays unaware. He will give you free stealth kills until he becomes self-aware.
Getting to know the map and flanking is whats important. Try to avoid combat as much as possible and if you see action somewhere, go around it and to your enemies backs for some possible stealth killa. Any campers you may see, go for them right away, you're silent so they will have difficulty hearing you. Another way is if you are in a fight, it is possible, albeit difficult, to run behind or roll behind your enemy and stealth kill them if you're quick enough, this is possible but chances are won't work, but hey why not try it. If someone is chasing you, use the power of deception, lol; run in circles but take uncommon routes so the enemy gets turned around and when they have lost you, pounce in from behind and attack.
These are things I learned fro leveling Cloaked to level 3 multiple times!