Snippet from an old script which deletes documents from a SharePoint Document library using SPWeb.ProcessBatchData(). Script used a CSV file of $activities which defined what should be deleted.

Delete-SPDocsInBatch.ps1

if ($clean)
{
	$deletedSharedDocs = $false;
	Write-Host "`n`nCleaning all lists of all content...Please wait"
	foreach($activity in $activities)	
	{					

			$web = get-web $activity.RootUrl
			$list = $web.Lists[$activity.ListName];
\			if ($list.BaseType -eq "DocumentLibrary" -and ($list.ItemCount -gt 0))
			{	
				if ($deletedSharedDocs -eq $false)
				{	
					Write-Host "Building Batch Delete Query...";					
					$strBld = new-object System.Text.StringBuilder;
					$strBld.Append("<?xml version=`"1.0`" encoding=`"UTF-8`"?><Batch OnError=`"Return`">");									
					$list.Items | % { $strBld.Append("<Method>");
										$strBld.Append("<SetList Scope=`"Request`">");
											$strBld.Append($list.ID.ToString());
										$strBld.Append("</SetList>");
										$strBld.Append("<SetVar Name=`"ID`">");
											$strBld.Append($_.ID.ToString());
										$strBld.Append("</SetVar>");
										$strBld.Append("<SetVar Name=`"Cmd`">Delete</SetVar>");
										$strBld.Append("<SetVar Name=`"owsfileref`">");
											$strBld.Append($_.File.ServerRelativeUrl);
										$strBld.Append("</SetVar>");
									$strBld.Append("</Method>");	
									}
								$strBld.Append("</Batch>");					
					
					Write-Host "Processing Batch Delete..."
					$result = $web.ProcessBatchData($strBld.ToString());
					Write-Debug $result
					$deletedSharedDocs = $true;	
				}				
			}
			else 
			{	
				if ($list.ItemCount -gt 0)
				{
					Write-Debug "Deleting : $list.ItemCount from  $list.Title";
					$list.Folders| % { $_.Delete()}
					$list.Items | % {  $_.Delete()}					
				}
			}
	}	
}