So I have been playing Borderlands 2 for a while and have compiled what i believe to be the best skill set when playing as the Assassin. Each character has three skill trees in which they can invest their upgrade points. For the Assassin, these trees are; Sniping, Cunning, and Bloodshed. The first tree, Sniping, should be fairly self explanatory. There are skills which improve your ability to attack from long ranges such as increasing aim steadiness and zoom, increasing critical hit damage, and even increasing bullet velocity. The second tree, Cunning, is for the players who believe that stealth is all powerful. Cunning has skills such as increasing attack damage from behind, the ability to mark opponents, and even throwing explosive kunai while cloaked. The third tree, Bloodshed, focuses on melee attacks, utilizing skills like increasing melee damage and maximum health, restore health from killing with melee, and the ability to redeploy your hologram infinitely as long as you keep killing with melee. A balance between all of them may be best for most gamers. For me, however, sniping is the way to go as it is extremely useful, not only with snipers, but many different guns.
Article from Gamersyndrome.com