I'd just like to add a few tricks to a few of them.
For fighting the Griever the 2nd time when you finally get to defeat him. If you stand behind the train car his laser cannot hit you. Once you destroy all the blue crystallization, toss the train car into his knee and shater that blue protector over his heart, and then move out of the way while he kicks it back and makes a small shock-wave. Run around to avoid the falling debris. When he has his little gnat friends come out, stand behind the train and just use the 'B' button to grab them and kill them without using your sword. You'll not only get a ton of health, but as he beams around with his laser he cannot hit you behind the car. Then toss the car back into his knee and attack his heart. Rinse and repeat. It's very easy and you should not lose any health or very little.
For fighting Straga, you say you must time going into one portal and out of the other just right for when he drags his hammer across the ground. Well, you can also go through normally and then just double jump to bring out your wings and War will stay in the air much longer allowing the hammer to pass underneath. Generally he will hit towards the portal you're at, so when he's about to do that, go into the portal and when you come out the other side use War's wings to have him stay in the air a bit longer and the hammer will pass underneath.