Mass delete custom records
This is a guide with screenshots that you can reference as well.
You can try the mass delete option that uses in line editing of saved searches. Create a saved search for the records if you are able to use inline editing, you can select multiple lines at the same time (similar to the multi-select functionality in Excel, selecting groups of lines using the CTRL or SHIFT key). Once you have selected the group you want to delete, on the far left hand side use the green button to choose delete record. Again, this is all dependent on being able to use inline editing on a saved search for your record.