A contractor has partially destroyed my retainer wall during excavation. Can a file a lien against him if he refuses to pay for the damage?
No, contractors who improve a piece of real property per the terms of an agreement, are entitled to file a lien against the owner of that improved property when they are not paid in accordance with that agreement. Because you have no agreement with the contractor or developer, you have no basis to file a mechanic's lien.
Based on the facts that you provided, you do appear to have a case for damages against the developer for negligence and (potentially) trespassing if they were on your property without permission. If the developer refuses to take responsibility and you think you can show that his or her contractor caused the damage, I would recommend that you consult with an attorney who understands teh applicable elements of construction law.