Behold the unholy necromancer!
A bit late reply, you probably have finished the game, but for the sake of continuing the disccussion and perhaps for future interested readers, I share my two cents.
To the OP's point, I feel like early game until early-mid act two is probably the hardest, and there are some tough fights late act 2 definetely, but I think act 3 is deceiving, because it's overall too easy, to the point where you progress it too fast and arrive in act 4 underleveled and maybe even badly equipped. Act 4 has mostly only really tough fights, so it doesn't help to be one or two levels below the enemies.
Overall, try not to fight on the enemies' terms, they usually have the positioning advantage. Also, the most obvious route to a fight is usually the 'lol, ur dead' route, so try to find an alternative route and how to use it to your advantage. But this is generally easier once you know what's coming your way, ie. you've played it once through already.
