There is an easy solution to this that doesn't involve any workflows or event receivers...
You can create a folder or series of folders and use location specific default values for your metadata column to assign the managed metadata terms depending on which folder they are dropped in.
Once you drop the item in the folder and the metadata gets assigned, you can then move the files out of the folders and the metadata terms will remain. Alternatively, you could just view the library with no folders, but the folder will still appear in the url path for the document, so I recommend just moving them out of the folder.
You can set-up "Location-Based Metadata Defaults" by going (from the ribbon) to Library -> Library Settings, then under General Settings select 'Column default value settings.' On the left-hand side you'll see a list of your folders under the heading 'Location to configure.' I will typically create just one folder and change the default value depending on what items I'm uploading, but you could create multiple folders for whatever terms you want to assign.
Once you have this setup, you can bulk upload files into the specified folders and they will automatically be given the corresponding metadata values for that folder.