I'm only level 4 on my first through but I have a strong RPG background.

A lot of the complains from the OP is good but not necessary.

For instance: The game was programed where regular enemy always aggro nearest target, why? so a fight won't get overwhelm by getting all your glass cannon butcher via focus fire.

Think about it. You'll die.

The game become easy because you play to much. It simple as that.

The second part is powerful skill books. I notice some of the NPC carry level 10 skill book where I really want to have but I can't because I'm still level 4. Just imagine if i could.. i would breeze through the game.

Since the OP offer constructive feebacks about the game being easy after level 10 and he played on hard... well that mean all Larian need to do is nerf some of the overpower skill books and make the player work harder and longer to win a fight. And if a player choose to play on hard mode, every regular fight should last for 15-30 minute long. Go higher and it should be in the 40-60 minutes for each fight but more importantly potion and healing spell should have a longer cool down.

Problem solved.

ps. I really want to see some kind of proof, like a video gameplay from the OP on what it mean to be easy for him.

I'm certain his damage will be extremely high due to some of the over powered skill books he use.