The Ninja maat fight is pretty basic. My static when leveling NIN was thoroughly surprised when I just randomly attempted the fight and won.
I'm not going to say it's without its challenges though.
First off, be sure you do the usual, sleep for TP (Opo-Opo Necklace + Sleeping potions, 100% TP is fine) and buy a TP wing. If you are in need of one, Arctyrael can synthesize one for you, however not for free.
In any case, AFTER you debuff Maat (please remove his shadows before attempting this, he will most likely put up Utsusemi: Ni as you enter the ring) use Blade:Jin and close it with Blade:Jin. This will remove a significant amount of HP. Stress evasion, this is about the only time I will tell a Ninja to stress evasion, because at level 70, subjobless, and without much haste gear support it can be difficult if he keeps taking your shadows. Ninja Maat will throw at you, he will use your Elemental Nukes, and worst of all, he hits like a goddamn mack truck. I got hit one time in my fight with him, for 227 damage. He will use Howling Fist. He may or may not use Asuran Fists. He -will- use Mijin. And it -will- hurt. The key thing, is to keep him completely enfeebled, and to not get hit. Bring a small supply of Hi-Potions, and also bring a Pamama Au Lait or two, depending on how fast you go. There are two winning conditions to the Ninja Maat. You either:
A. Evade X number of times (this includes Utsusemi, I don't want to lie to you on the number, since I honestly don't remember, but this is why I stress evasion)
B. Remove 100% of his HP.
I won when he was at 15% health, from evading enough hits. His Mijin hit me for around 760. It almost sucked.
But yeah, compared to a couple other jobs, this isn't really too bad. Enjoy it, because it is pretty fun.