I would recommend that during your regular playthrough you focus on picking light options in dialogue and using light cards in your deck. IMO the dark cards are better, and it will be easier to farm the dark points you will need at the end of the game.
Difficulty does not affect anything meaningful in this game, so feel free to play on story for slightly easier battles.
Throughout the game there will be dialogue choices that have either a white or black circle near them, picking a choice with the white circle will give you 1 point of light and picking the dark circle will give you 1 point of dark. Pick one of them and stick with it the entire game, if you go back and forth it will make the grind at the end more annoying.
Another way to earn light and dark points is by playing cards of that type. At the bottom of some of the hunters cards there will be a white or black circle (you can also click more info and it will state at the top whether it is light or dark). Using 4 of these cards in one battle will give you 1 point towards light/dark, with a maximum of 2 points per battle.
Upon reaching 200 points of light/dark, you should start picking the other option. By the time I was at the end of the game, I had the light achievement and had JUST made it back to neutral. If you are still like 50ish points away from neutral, it would probably be faster to start new game+, which will reset your light/dark balance, but you will not carry over a lot of things like cards unlocked, but you will be able to pick all the dialogue choices again.
If you are close to neutral, or have even made progress to the opposite affinity, you will want to do the T.H.R.E.A.T. room with hunter every day. You can easily play 8 cards in turn 1, and you can have a card that gives you conceal to end your turn so you dont have to bother killing everything. Just make sure you play 8 of the matching affinity cards in each battle. I did dark as my second affinity and the ability Last Sight has conceal on KO, which was nice.
I usually picked a "defeat nest mother" mission for the daily mission, as i could win on turn 1 most of the time, but i always tried to play at least 4 affinity cards for the extra point. Also, make sure you check any heros that have a map marker on them, as you may get conversations for extra affinity points as well. This is really the only grindy achievement in the game, and its not too bad once you understand how playing cards earn you affinity points.
The hunter deck I used was:
Wild Strike x2
Mindbender x2, exhaust helps the T.H.R.E.A.T. room be a little faster
All Out x2
Getting ability mods like Extra Heroism Generated, Quick, Reduced Heroism Cost, Draw Hunter Ability, and Extra Damage to Full Health Target (any of these with +100% offense can be nice) can help speed up battles in general.