What would be the correct way to level a basement floor that is this much uneven, so that I could build a sublfoor consisting of delta-fl and plywood ?
If you plan on using dri-core then you don't need to level the floor with self-leaving concrete or any similar method. Instead, you'd just shim the dri-core panels to make them level.
If you are just going to put down standard OSB then you'd want to use self-leaving concrete.
A big advantage to just shimming the dri-core is that most basement floors slope towards a drain and with shims, you'd maintain this slope so any water that gets beneath the dri-core would still drain properly instead of sitting underneath the OSB. And of course this is why dri-core has a rubber bottom layer.