So, like, how do you Ninja Gaiden?
Not sure if the way you wrote that was intentional, but I love this question written this way. xD
The Razor's Edge demo was the first time I've ever played a Ninja Gaiden game and I can't get anything done for the life of me. A simple crowd of regular enemies decimates me and I have no idea what to do? I want to unlock Kasumi ;___;
Honestly, the most important thing is understanding the flow of combat. That will change based on the enemies present, but knowing what you're up against and how those enemies react to your movements, attacks and defense is critical. After you get that down, there's actually a lot of wiggle room for improv. and experimentation (at least on the earlier difficulties).
Try different things. For example, sometimes enemies will do stupid shit while you're doing an Izuna, leaving them exposed for Izuna-chaining (after you drop one guy, immediately do XY and you can scoop up another). This doesn't always work, and you have to account for position, timing, and their current frame stance. But, after trying it enough, it will become second-nature. When you land, you'll immediately know whether you can chain another one or if you should take other action without even thinking about it. Same goes for "When can I land a guillotine throw?" and other things.
My personal method for experimenting is by utilizing the head pounce (if you have that unlocked). This will allow you to stun and skirt enemies safely while observing the behavior of the others in the group. It will also let you temporarily immobilize some hyper-aggressive asshat who is determined to prevent you from toying with the other enemies on the battlefield.
TL;DR: Instead of fighting for your life every second, suspend your will to win and become inquisitive. Fuck around with your enemies for a bit like they're lab rats, observing what they do when you do things you haven't done before. Try new combos to discover their independent lurch, speed, stun and guard-break properties (like Jinran XX->XY), and note how/when enemies attack and how that differs when they are in groups (and the group they are in does change their behavior drastically). After that becomes intuitive, re-approach the fight with the intent to win. You'll be amazed the difference it makes.
Hope that helps you some.