The thing about environments is that the slight opacity is actually a good thing. I use a 60-90% opacity brush and blend by choosing a color, putting it on the canvas, and then sampling from the color created, several times if I want a more mixed version of it. The bleed allows better mixing and giving an overall unity to the color scheme, something which is generally hard to achieve if you're using 100% opaque colors.
The brush engine he uses at .22 looks like a standard circular brush with no shape dynamics and a lower than maximal flux (probably between 5% to 20%). It's not a precise setup that allows you a lot of precision, but that's not the point - the point here is speed and efficiency to set the stage. He's probably got a Touch Ring set to brush size to allow quicker adjustments.
The brush he's using at 9.00 looks rather similar - I get the feeling he mostly plays with flux ("the amount of color deposited per unit of movement") and doesn't use pressure for opacity.
From a general point of view pressure for opacity isn't fundamentally bad but if you want opaque colors to be able to move things around I'd advise you to first create an opaque shape on whatever layer you're working on, lock opacity and *then* start working with pressure for opacity. That way, your shape will look the same no matter where you move it since it's opaque.
You might also want to look at clipping masks and blending modes for layers. Also adjustment layers. Just random but potentially useful suggestions
Yes, I was referring to flow (the name didn't change I just wasn't careful enough). Flow and opacity have roughly equivalent effects. To be pseudo-precise, opacity affects the whole stroke (for instance, 10% opacity means your stroke won't get any more opaque than 10%), whereas flow affects the painting brush (the amount of opacity you add to your stroke per unit of time/distance, depending on your brush engine (I think the airbrush toggle is key here).
RynkaDraws's remark below (about lower than 100% opacity for blending) is excellent. If you try and apply the method with fully opaque objects, I advise you to also find a way to blend these objects with the background (because you won't be able to use the less-than100% method then). It could be by adjusting the edges blending once you're relatively done with the object, or you could, for instance, work on the background around the object or create a new layer (probably in overlay, multiply or screen mode depending on the kind of light you're dealing with) to take care of global blending, with a low overall opacity (think of it as a fog factor).