Ugh the 5 star enemies are horse shit to fight. They dodge everything ad parry everything and it's horse shit fighting a dozen at the same time. I have the strongest armor and everything in the game and still die in a few hits, it's complete and utter horse shit -__-.
Enemy behavior depends on enemy type, not level. Level just changes health and damage, as far as I can tell.
Average build guys might parry some attacks, but generally don't dodge, and all of their attacks can be easily parried.
Slim guys parry a lot of attacks, and dodge gunfire when not stunned. They also can and will dodge out of a regular attack when they are grounded (though not out of gunfire). They throw bombs that blind for a brief bit of time, and tend to attack three times in quick succession, with the last attack being the only one that you can parry to set them offguard. They can also be caught offguard by the staggering strike, and don't dodge charged/power attacks.
Big guys can't be countered off their guard (though you can still counter their regular attacks to prevent them from hitting), and they can't be effected by staggering strike (using it puts you at a disadvantage) but are slow, can be interrupted by regular attacks, or by gunfire, and they are susceptible to charged attacks. They perform a special attack that grounds you and can't be parried, so you'll either want to roll away or interrupt it with an attack of your own.
Riflemen are weenies who attack with basic and easily counterable attacks. They are susceptible to anything, but they have obnoxious vision and high damaging, ludicrously accurate rifles (they also don't seem to show up on your conflict radar 100% of the time >:[ )
Guardsmen (kind of largish guys who aren't quite as tall as big guys and are usually standing guard somewhere) parry almost all of your regular attacks, even when they rear up for an attack themselves (though that does interrupt said attack). They'll catch a staggering strike too, unless first caught offguard by a successful parry. They are succeptible to charged attacks and gunfire though. They perform a special attack that knocks you off your feet, and it can't be parried, so you'll again want to interrupt it or roll away.
Unless I'm forgetting, the regular guards only come as average build guys and slim guys, at least on the streets. The red revolutionary agitators come in all types.
A few more tips:
If you know that your next attack will send an enemy into the "finish him" wobble, you can just hold the attack button and you'll do a finisher outright. This negates all defenses that the enemy might put up.
You can parry out of almost any situation, even if it looks like you can't. In mid-swing, during a finisher (causing the finisher to continue without Arno's presence), immediately following a roll, even in the middle of another (successful) parry. The same goes for rolling. In addition, the game doesn't always successfully display an enemy's life/behavior bar, leaving you a little in the dark about when to parry. However, that doesn't mean that the attack is unparryable, you just have to watch the animation itself and time it right. There's generally a pretty big window. This isn't Demon's Souls.
Don't fight too many enemies at once. If you can't keep track of them all, use evasive maneuvers and run. Try taking a few out with cover assassinations.
And lastly, every enemy is susceptible to bombs. Of course. *sigh*.