As per the documentation the [CustomField][1] metadata type does not support wildcards sadly. You can access **Custom Fields** on **Standard Objects** by using the **CustomObject** metadata type as described [here][2] against specific Standard Objects. Then parse the .object file to build your **destructiveChanges.xml** file. Finally you can also use the [sf:listMetadata][3] task to download a list of all custom fields and then filter locally for Standard objects.

		 <sf:listMetadata 
			username="${sf.username}"
			password="${sf.password}"
			metadataType="CustomField"/>

You might also be interested in an open-source **undeploy** Ant target published [here][4], that wraps most of what i suspect your trying to develop up into a single Ant target. Sadly Salesforce do not yet give us a 'clean my org' task, so we build our own!


  [1]: http://www.salesforce.com/us/developer/docs/api_meta/Content/meta_types_list.htm
  [2]: http://www.salesforce.com/us/developer/docs/api_meta/Content/manifest_samples.htm#manifest_standard_objects
  [3]: http://www.salesforce.com/us/developer/docs/daas/Content/forcemigrationtool_listmetadata.htm
  [4]: https://github.com/financialforcedev/df12-deployment-tools#using-the-build-file-targets