Ori and the Blind Forest

So, for getting all of the skills in the Ability Tree, you will need quite a lot of experience orbs (the orange things you either collect as stand alone orbs or you get them from killing enemies).

I suggest you just play through the game and get as much as possible of those as you can, but even with that, you might end up missing a few Ability points.

Now, the important thing is that you get the final Ability point before you enter Mt. Horu because thats a point of no return.
I suggest you copy a save to a different slot before you enter it, in case you want to grind it later.

I found a perfect place to grind the remaining experience orbs. Its located in Hollow Grove, right before you make the jumps needed to get to Mt. Horu. In this area, there are two enemies that give a lot of experience orbs and spawn instantly as you leave the screen.
You trigger the green rolling guy on the right and just jump away so he kills himself, go to the left and using bash ( + ), kill the shooting orb with his own projectiles (two hits kill), then rinse and repeat. This method seems to give out the most ability orbs for the fastest time.

The way to do it and the location can be seen in the video I made:

This achievement is for getting all of the abilities in the ability tree, in one playthrough. You can do this by collecting the orange ability orbs that are scattered around the map as well as the white-ish orbs that are found in more secret areas. The white orbs will give you a full ability point instead of partial points as the orange orbs do. You will need to collect 50 ability points in order to fill up the entire ability tree.

There are not enough ability orbs (white and orange) on the map to fill up your entire tree, so you will need to get a lot (probably about half) from enemies.

If you're like me and you waited until the last minute to try and get them all (I got to Mount Horu needing 6 more ability points to get my last two abilities in the tree), I found a pretty good place at the top of Mount Horu, where you can take out a few of the shooting orbs. As you can see in the video, you start at the map stone and jump out, there will be a shooting orb. I use it's first shot to propel me higher, then it's second shot to kill it. I then float down and use the lower ones projectile to kill it, projecting me up enough to reach the map stone again. Run to the map stone, which will spawn the right most shooting orb and repeat. I use the first shot to propel up, so that the lower most shooting orb will respawn off screen. They must go off screen in order to respawn. You get approx +100 damage points from each kill (2 xp orbs, +40 and +60). This is still grindy, but I think killing these two, makes it go a bit faster. Also, this is one of the last places that you can do this.

Any questions, let me know!

Farm experience orbs while being idle !

I won't go into all the details like the other solutions, you need these orbs.
At this spot, you can farm the remaining experience orbs you need to unlock all the skills. Just sit there and relax. Don't forget to save before leaving your place.
If the controller turns off, the game will pause.