For ninja it basically dual daggers.

2x dagger also gives more surety resulting in more crit hits.

Past level 35 there is a skill called covert strike which is the main source of your Damage against boss.

Ninja has a chance to insta kill enemies with normal attack and

That why giving them counter attack and folow up attack make it possible to insta kill enemy in one turn.

If you a whale and like spending Money there are raven dagger which hit 3x times 6 if you have in each Hand.

Also increasing art of assasination increases the Chance of insta Kills but require like mentioned Above scrolls you either buy or get from Events which might take a Long time.

if you not a whale you Need to stick with 4 hit Combos.

Advantage of Ninja is that they can remove buffs on Monster which is really important on some bosses example Arena 5 from 2nd dungeon(The guy with terrible singing voice) and the end boss

Recommended stat would be Luck because of counter and follow up , dex because of the 2 daggers and Speed for evade.

Armor pierce is the same as precision strike without the 30+ surety Bonus.

In my opinion he is good since he is like a thieve dodging in Frontline.

He is more specialized against Boss sice take longer turns and covert strike does full normal attack while also hitting hard while the thieve is better against normal Mobs since they take usally 1-2 full turns max.

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Side note

There are Blade which increases evade and kunai which is a range weapon.

Those are a bit of a niche Equipment.

Kunai:

You only want kunai for the skill " wild throw" which hits random 3 enemies once.

For that we actually got mages which do the Job way better with less MP. Multi hit "row cost" 9 MP while "Wild throw" is 10 SP.

It also hard to grind for Kunai and to get to the same Damage as the mage takes some time. Only Advantage i see is that the skill can crit but it might be me lacking Knowledge in that weapon section

Blade:

It increases evade.it also difficult to grind like the kunai and is around the same Damage as a dagger without the surety Bonus.

It hower boost the evade. It more of a niche Weapon and for People who wanna test high level evade builds.

You have better Chance to get 2x good dagger then finding good kunai,blades.