I need a script to split a large XML file

I have an XML file from a provider with about 125000 products which is hard to import and update for the shop.
I need a script that will split this large XML file into smaller files based on the main category.
Xml looks like the attached:
I want a separate file for each of the MAIN CATEGORY.

So in essence, The script should be able to read through the XML file, split the category tree and create a file for each main category.
Note: This large xml file is auto updated so i will need to be able to run the script via cronjob and make the separate xml files and then use those files to update products. (this is sorted, so i just need the script to split the files)