After an assessment of the board, you can usually tell how hard or easy it will be to take our your target. I enjoy assassin games, and do win my fair share.
If your target happens to be close to you, then it's easy. Just take out his armies as fast as possible. One or two territories per turn.
If he's too spread out, then it's beneficial to try and grab land, going for bonuses, while keeping yourself from being targeted because you're too weak to defend yourself.
Secrecy is of the utmost importance, though.
Once your group gets wind of who YOUR target is though, if you are too quick, they may move to block.
I find that sometimes it's necessary to use a feint or two so it's not certain who I am targeting. If you can take at least 2 territories a turn, and sometimes your attacking someone else, it's not obvious who your target is.