Rototilling roots is a pretty tough chore. I would not put down any herbicide as such products either would not work on the roots (only if sprayed on the foliage) or is they did have root effect would damage trees that have their roots in the same area.
Try rototilling but be ready to stop the tiller a lot to pull/dig roots that show up. Many will be from trees, even some distance away.
When the soil is well worked mix in some composted bark as you form planting beds. You may also want to purchase a soil/compost blend to help create slightly raised planting beds to facilitate drainage.
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