Can I us overpowering Assault to destroy my opponent's terrain?
Relevant Text...
Overpowering Assault Battle: Choose your performing unbowed Commander and bow his performing Follower: Destroy a target enemy card without attachments.
Overpowering Assault vs Terrain.
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Can I us overpowering Assault to destroy my opponent's terrain?
http://rules.l5r.com/CE_Glossary_E#Enemy: "In a phrase such as "an enemy card," "enemy" does not refer to cards outside of units(.)" Emphasis added.
Terrains aren't in units, so: No.