Well, not sure I'm an expert, but hopefully I can help a tab bit. Tall order. Here's the most simple way I've found to beat most bosses.
This will mostly cover strategies for Master Ninja because usually what works in MN works in Normal mode, whereas what works in Normal mode doesn't always work in MN.
Also, I make a few mentions of the Sigma games, but mostly stuck with Black/vanilla NG2 because Sigma 1 sucks and I was too lazy to do Sigma 2. If you want any additional info on sigma-exclusives, feel free to ask.
Ninja Gaiden: Black
Murai: Either use the vertical wall climb Y slam or running X
Horseman: Kill all magic guys. Then go to far end near gate. Wait until he passes (hold block) and attack. Can also be done by charging a UT in Black. Listen for him coming as you won't be able to see him (off-screen).
Fat Guy: Couple slashes, he'll try to grab. Roll away (but not too far) and continue attacking. The hardest part of this fight is getting in to begin with, as sometimes the awful camera can making dodging his blasts awkward. Always try to stay in close. His attacks are slow, few and unsafe. At range, however, he's a monster with camera abuse.
Hyracubus: Not that anyone needs help on this chump, but use flying swallow, windmill shuriken, w/e to knock out his tentacles, then use either XYXXXY or use the essence to charge a UT.
Fiend Ryu: Depends on weapon. Lots of tactics work, however. Some include Izunas or the Dabilharo.
Bone Dinosaur: XXX his toes, dodge his attacks. Avoid flying swallow (imo). Running X can also work in some situations.
Alma: Flying swallow XYXXXY. Izuna for shadow fiends.
Tank: Fucking I don't know. Shoot him with the bow, I guess.
Helicopter: Same as tank.
Worm: Easiest boss, imo. Just block/dodge, roll through his sweep attack. Then use the Dabilharo XXXXXY attack. Same thing for fire one, except jump to dodge his flame circle.
Bone Pterodactyl: Flying swallow between his legs, or use a short sword combo. When he bows, use the sword XYXXXY or the Dabilharo XXXXXY
Doku: Vigoorian Flails XYY Equip the horse armband for extra measure
Yotunfrau: A very fun boss. Just prance around and do any combo you like after he tries his giant sweeps. Use Izunas to mop up the Bast fiends.
Smaugen Dragon: Jump to the top right (his left) and prance back and forth until he tries to bite you. Then (you guessed it) Dahilharo XXXXXY
Alma 2: Same tactic as before but with a less retarded camera (usually). Basically the same tactics apply though she will block more and you may want to consider Y dashes. Similarly, your XYXXXY combo won't work the same. Consider other powerful combos XXXYY or 360chargeY etc. Assume she will do her grab every second (because she will).
Ghost Doku: I'll admit, I cheat and use ninpo to swat off the ghost fish. I get sick of their shit. I hate this fight because the camera is ass and collision detection follows in the same vein. Essentially, keep combos short. You'll probably want to side roll because of weird things with his grab's range.
Nicchae: Same as Alma 1 except with the worst arena you can possibly imagine. Hopefully by now you have the Unlabored Flawlessness and can use that.
Ishtaros: The hardest boss, imo. Best combo I could find is XXXY. Her strangs are generally 2 strikes, guard break, unblockable attack (iirc), so note when you're blocking will fail you and dodge then. When she starts to whip areound the arena, charge a 360 Y UT with the TDS and unleash it right when she jumps on you. Fire wheels is very useful if you're okay with using ninpo.
Marbus: Like Alma but easier. Izuna shadow fiends. Wait until he spins, dodge, and use your typical Alma combos.
Vigoor 1: Use the 360Y. AS far as dodging things go, just try and dodge or something. Always hated this part.
Vigoor 2: Spam all arrow types and incendiary shurikens. The explosive arrows will be the most effective for destroying projectiles, and the core arrows will deal the most damage. If you can't dodge the giant head (go through it/forward) then feel free to ninpo i-frame it (end of level).
Murai: Honestly, I never fight him legit because I always have a hefty amount of health packs here and see no reason to not burn through them. I wouldn't imagine he's too hard, though...
Ninja Gaiden II
Rasetsu: Huge difficulty spike here. I find if you upgraded the lunar you'll have an easier time than the DS. Izuna the IS ninjas if you have the space/time. Rasetsu is aggressive, though, and will likely require your full attention. The lunar is nice because if you attack normal enemies with it, the sweep will often hit Rasetsu and slowly chip away at his health. 360Y at the start, and use mixes of XYY and XXY for the ninjas and rasetsu. Anytime you can adsorb essence for a successful UT, do it (especially 360 one). Second fight you can use the Flying Swallow->Izuna Falcon's Talons for IS ninjas if you like (jump forward YXY) though honestly lunar is probably safer. Consider de-limbing the IS ninjas without killing them to make them walk slower (but spam just as many shurikens).
Genshin: Dumbass camera will be 100% of your woes (better on Sigma 2). Block, and try to dash though the final attack of each of his strings. This will leave you close up for some guaranteed attacks. In Xbox, use XYY with the DS. In Sigma 2/RE, use XYXXY Lunar. Use the guillotine throw to mop up ninjas in first fight. Izunas later (though only use them at distance, or else he'll flying swallow you out of the air during the xyXXXy portion).
Gigadeath: Sigh. More bow and arrow crap. You can technically shoot him, wait till he comes near, charge up a UT (360 lunar?) and tear him down that way. Be wary of everything 'cause he's a bitch, just like the camera.
Blind Ogre: Dash under his legs and chop away. If he spins, dash behind him. If he pounds, dash behind him.
Alexei: Someties he will spam his tornado. All-fucking-day. In that case, go to your happy place and accept that you can't do squat. Generally, you can avoid this by staying in close. Like Genshin, try to dodge the final strike by dashing though rather than away. Flying Swallows can work but running X is better to get in. Lunar XXY is your best bet.
Water Dragon: Don't jump in the water. Use the bow/cannon (Sigma 2) to shoot it's head from the platform. Stay as far to either side as you can so that you have room to dodge the sound ripple attack when he uses it. The crystal spikes are the hardest move. Just learn their timing. Usually dash-pause-dash-dash (iirc)
Bone thingy: I hate this damn bastard so much. 1 hit kill with stupid distance and sweep, plus an army of annoying spider bones. Get in, and either spam UTs (use bone spider essence) or use some combo that gets you high to his head (lunar/dual katana XYXXY/XYXXXY). When he falls, give him everything you've got. If you're lucky, you'll stun-lock him and kill him the first time he falls with loops.
Volf: Almost all guard breaks, unblockables or throws. Sometimes you can just flying swallow him to death. Usually, you have to time your dodges up close and get him with your standard "safe" combos.
Armadillo: Stun-lock this bastard's head with incendiary shurikens. In Sigma 2, things are mosr difficult. Camera's ass, boss is cheap. Scythe jump XY to the head is risky but rewarding. Hope the camera doesn't fritz out on you. Block when he is killed.
Zedonious: In the xbox, jump XY with the scythe until he's dead. Use lunar Izunas or Art of the Inferno to eliminate Gajas. Very easy. Sigma 2: much more of a cunt. Emna's fang single strikes tend to be he safest bet. Some lunar tricks work too.
Giant worm: Hide in a hole and charge scythe UT. When you hear him coming, let go and repeat.
Flying dragon thing: Piercing void. Two hits. Get a two-in-one if you like. Otherwise, spam jumping arrows.
Elizabet: On xbox, same tactic as Zedonious. On Sigma 2, dodge through final attacks and stay and mid-range. Lunar XXY or UTs if you feel risky.
Dagra Dai: Same as Zedonious. Flying Swallow the Van Gelfs to death, but leave one alive so he doesn't re-spawn them (preferably without a limb). Same tactic for Sigma 2.
Archfiend 1: Arrow to head, double jump dodge the blood, arrow to stomach (or piercing void). Be fast to prevent him from spawning those annoying head things.
Archfiend 2: Go to his hand and use lunar XXY. Riskier: go to head, use scythe jump XXY. When walking, use lunar XXY
edit: I was going to go to sleep but then decided talking about Ninja Gaiden would be more interesting. Now, at 5:15 AM, not sure I made the smartest choice.
