Well you can certainly forget about the tree generator altogether. You'll have to build it yourself. I would use the vertex modeler. One thing you can try is to make a long bent-up bough and then break pieces off of it to construct the various parts of the tree, so you don't have to start each branch and root from scratch. After that, great textures will be the key. You might be able to do the leaves with a texture, so you don't have to model them. This is true in C4 especially, because you can use the alpha feature to cut a single polygon into a spray of needles (or whatever those are), which would save a lot of modeling.