Topsoil would be best..

Compost was suggested to break up the clay soil by mixing it in. It would have to be the right ratio then added to the topsoil to reduce the amount of topsoil ordered.

You don't want compost around the base of a tree if that's what you were thinking, and not plant anything in pure compost.

I would dig a hole first and front it with boards on three sides as a hybrid rasied bed/planting hole.

Intsall a french drain at the bottom of the hole so any water trapped by the clay can drain out onto the hillside.

If you don't dig a hole, drill holes at the base of the boards for drainage.

I think you need another landscape engineer. Someone who can solve the leveling aspect. We don't know the dimensions you're working with. Slope angle, width of slope and current drainage pattern. Does it need a retaining wall or do you have space to dump a drainage mix of topsoil to create a berm. And then you'll need to protect the trees in the same fashion you're using already. Compost comes into play then. Not in the leveling process.

trace, thanks for your feedback and suggestions. I'm having a bit of trouble visualizing exactly what you mean by the hybrid raised bed. Do you by any chance have a link to a picture or website that describes this sort of thing in more detail?

Dan, yes I am trying to set up consultations with some of other landscapers to hear other opinions. I don't think this guy fully grasped what I wanted to do, so I'm explaining it at the outset now that the trees need to be planted in a certain way (raised bed, hardware cloth) to thrive in this environment. .

Here is my idea of a hybrid planting hole/planting bed in sketch.