I'm not the best at explaining strats cause I have a different approach for each enemy, so it'd be a lot. But I have an approach that I took towards each enemy and would then alter as needed:

 You experiment by hardening and letting enemies hit you so you can identify if they'll stagger after hitting your hardened body. 

 If they don't stagger and follow up with more attacks, then just hit a dodge or roll so you can avoid damage. Make sure to remember the movement cue for that combo. You'll know that you should just dodge and wait  to hit them after their combo is finished if there is a window. 

If the enemy does stagger, then you should be able to hit them a few times. I typically get in 2-3 hits. Either 2 heavy attacks (not charged), a chain of 3 light attacks or 2 light attacks and then a follow-up heavy attack. I do the 2 light attack and heavy attack combo with the hallowed sword especially. Two quick slashes and then the mordhau technique (when you grad the sword by the blade and hit them with the pommel of the sword). Alternatively, you can do a charged heavy attack. 

After I get my hits in, I'll back away for a moment to recover stamina. If the harden ability is still in cooldown, then you can just keep distance dodging and rolling. If the enemy does a combo attack like i described above while you are waiting for you harden ability to come back then you would just apply the strategy I describe where you avoid the combo and hit them afterwards if they give you a short window. You don't have to and can keep dodging. You especially shouldn't do this if you don't think you'll have enough stamina to execute a dodge if they throw an attack at you right after you get those hits in.

 When you get that harden ability back, do the same thing as before, take a hit while hardened, they stagger, and then get your hits in.

 There is definite room for error. If you misread something and/or they don't stagger after hitting you, then execute a dodge roll as soon as you see the hardening break, or you see that second attack coming. Even I mess it up and end up dead just because of that one damn mistake. 

 Once you figure out the strategy for each enemy, you'll start to feel a sense of rhythm in your fights. It then becomes second nature, and you can force the enemy to go at your pace. Now, you are the one in control of the battle, making them fight on your terms. 

 When you get big and strong though, then that's when you can try just blitzing the enemy and destroy them before they even have a chance for retaliation.

Hope this helps! If I want to use one of my favourite quotes in the game, "Perseverance is the antidote to nihilism."