So I found a neat and easy work around for exporting batches of items into XML format. I've seen that this has been a bit of a problem and some people are looking into more complex plugin development and updates. But if you are just looking for a way to export moderate sized batches rather than exporting each individual item there is a quick little fix that only involves copying and pasting a little bit of code from admin/themes/default/items/show.php to admin/themes/default/collections/show.php.
In the Items show.php on the admin side, there is a small bit of code that pulls up the various "Output formats." The code I am referring to reads:
<div class="panel"> <h4><?php echo __('Output Formats'); ?></h4> <div><?php echo output_format_list(); ?></div> </div>
I decided to just try copying and pasting this segment of code into the Collections show.php file in the admin folder to see what would happen. I placed the text directly before the final closing </section> tag. After doing that, I went into my Collections from the admin side of omeka and it generated an "Output Formats" section in the panel with the options "omeka-json" and "omeka-xml." When you click "omeka-xml" it generates a complete xml document of all the relevant metadata for the entire collection AND each individual item in the Collection. (I have not tried the JSON format because for our project we only needed to grab the XML)
Now this is a limited fix, because it requires you to put the items you want to batch export into a single collection (or multiple ones). Larger Collections also take a lot longer to export/generate an XML document. But it is a quick little work around that allows you to easily export the XML for collections up to around 500-600 items at a time rather than just individually exporting item by item.
Hope this helps!