I've just bought this game myself, but I've been playing Ninja Gaiden for a LONG time and even I'll admit that the difficulty curve for a new player to the series is like running head long into a brick wall.
The game's system is very different from your usual Hack-And-Slash type game, with Ninja Gaiden, if you make one mistake, you WILL be made to pay for it, as far as tips go, for general enemies, two words I shall offer you.
Flying Swallow.
This move has to be the greatest for dispatching your general enemies quickly and making you basically untouchable at the same time, if you get overwhelmed, feel free to use Ninpo, don't feel like it's a precious thing because that's what it's there for, granted it does little against bosses, Ninpo is really there to help you out of tough situations.
But yes, Flying Swallow is your best friend and it always will be, I'm assuming you know how to use it, if not, jump towards the enemy and press Y.
I'm not that far into Ninja Gaiden 2, only just got it today, but I find it plays like the Xbox originals, the first boss I took down in literally about 10 seconds, one fully charged Ultimate Attack followed up by a level 1 attack ends that fight.
I'll also say this much, the Dragon Sword is also your best friend, basically every other weapon pales in comparison in terms of how easily you can take down enemies, sure you've got things like the Scythe, which is powerful, but slow, the Dragon Sword should basically be at your side as and whenever possible.
Hope I helped, if only a little.