I'm a PLD main, so I'll state what I did for leveling my NIN sub. By no means is this the best way about it; nor are anyone else's opinions, no matter what they say >_>
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Cast your shadows early, and if you can, cast Utsu before your last shadow disappears. Post-37, keep in mind that Utsusemi: Ni overwrites Utsusemi: Ichi, but not the other way around.
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EXP with a "co-tank" (ideally, a 2nd NIN or a WAR/NIN with Utsusemi and Provoke, but basically anyone with Provoke) that you can alternate hate with: Assign a "1st voke". Tank 1 provokes off of the puller. Once their last shadow disappears, Tank 2 provokes, giving Tank 1 a chance to cast Utsusemi. Once Tank 2's last shadow disappears and/or Tank 1 is finished casting Utsusemi, Tank 1 provokes again, and the cycle repeats.
-From what I've heard, Elemental Ninjutsu: Ni is MUCH better than Ichi. If you want to quest for/buy the scrolls for Ichi, by all means go ahead, but it's not as effective as it's Ni counterpart in terms of damage. As for your Enfeebling Ninjutsu, they're effective in helping keep hate, but I've never really needed them pre-37. And if you're in a party, your RDM should already be enfeebling. :x
-If you like, you can level your Throwing skill and add in RAcc/RAtt equips for shuriken, but optimally you don't want to waste gil with Shuriken. If you DO have gil to spare,
Shuriken are the best DPS throwing item out there, but no one has really proven that they've kept hate just by throwing stuff at a mob. :(
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ARMOR--I went for the Evasion gear route, since you don't get much Enmity gear at earlier levels. DD gear is also nice, but think about it:
Cumulative Enmity Loss = 1800 x Damage Taken / Target Max HPIn other words,
you lose enmity each time you are hit. If you EVADE a hit, then you don't lose enmity. NIN already has an A- in Evasion, and stacking evasion will only make it better. Also, there's more
EVASION GEAR out there than DD at earlier levels:
Bone Harness +1 set
Beetle Harness +1 set
Federation GiKingdom TrousersTraveler's/
Nomad's Mantles
Empress/Emperor HairpinDodge EarringsSpirit Torque(I HATE HOW NIN'S CAN'T EQUIP
BATTLE GLOVES D;)
The rest I just filled in with
Accuracy and Attack /
STR/DEX equips, which I macro'd in especially for WSes (STR/DEX modifiers for Blade: Retsu and Jin, the two WSes you'll use the most
I THINK; correct me if I'm wrong, please!):
Royal Footman's GlovesWindurstian Tekko/
KyahanPhlegethon's TrousersSpike Necklace---POST 37---
Ochimusha's/Ochiudo's KotesJujitsu GiScorpion HarnessAlso, at later levels you definitely need to stack
Haste. Haste lowers your casting time for Utsusemi: Ichi/Ni, which is an enormous help, more so than stackign Evasion or Attack, in the case of tanking as NIN:
Fuma/Sarutobi KyahanWalahra TurbanDusk GlovesSwift BeltAs for
WEAPONS,
ideally you put the HIGHER DPS IN YOUR MAIN WEAPON SLOT and your LOWER DPS IN YOUR SUBWEAPON SLOT.
Fukuro--Dual Wielding these helps with your WSes, seeing as the modifiers for the most effective WSes are STR and DEX.
Nikkariaoe is also an amazing weapon that... well,
hopefully works at the right times while you're casting Utsusemi. OH, and for more Spell Int. Rate down, get a
Willpower Torque for cheap--Macro it in when you cast Utsu, or replace it with a Spirit Torque at 38.
My lists are by no means complete, the items are just what I could think of off the top of my head.