I'm loving it so far. My demo gamesave pulled into the full version with all equipment and qi intact. I ended up going back to that first level and trying that first stupid ass boss fight. What I can say, is make sure you have found all the battle-flag dealies on the battlefield before trying your hand at the boss (there are two rows of numbers on the top-right of the screen that tell you how many of each flag you have found.

Then make sure you have practiced the whole deflect mechanic (side note: I found that it helped me to also hold the block button down even while deflecting) The first part of the boss fight can sometimes turn into a quick pattern of block his first attack -> deflect his second -> then retaliate with X, X, X Y (for anyone using an Xbox controller on their PC). This should pretty quickly get him into a qi-depleted state where you are given the signal to do a powerful Y spirit attack.

The second half of the boss fight is a little more difficult, but I generally try to attack a few times and then wait for the red unblockable attack from the boss. If you can deflect it, you can go to town on him. A few of these and he should be at half health. At this point, your partner should prompt you to launch your Divine Beast attack...ending the fight on a cutscene.

Note: maybe stick the spoken language on English for this fight so you can hear your partner yell to launch your Divine Beast. The first few times I had it in spoken Japanese and missed the subtitle at the bottom in the middle of the action.

The first boss was by far the hardest for me.