Train ninjitsu off a ninja first. That gets you to about 30. Ninja's can be found in the tokuno city. Animal form a rabbit over and over, till you get to 40. Mirror image till you get to 65 or so, maybe 70. Now here's where it gets interesting.
Get a golem. If you're in a guild, you have to be in town for this to work. If you're not in a guild, then you only have to be in trammel. Most importantly though, regardless of guild status, you have to be in trammel.
Tell the golem to attack you "all kill." Remember, if you're in a guild you have to be in town, otherwise, if you're not in town, the golem will hurt you. If you're not in a guild, this doesn't apply. Now all you do is just focus attack(make sure you don't have a shield, you can't focus attack with a shield in hand). You dont' have to target yourself or anything, just focus attack, you'll get gains.
Once you get to 90 or so, switch it to deathstrike, and you'll get all the way to 120 this way. It does take a while, don't get me wrong, but you'll see gains. Took me a week and a half of training to 120 ninjitsu, and this was like, 5-6 hours a days of training.
Thanks to Mercenary for this guide!