i don't think you'll find an easier way than one by one on your iPhone.
If you sync your contacts with iCloud, you can access them on the web here
https://www.icloud.com/contacts/
if you go into the Contacts section, you can select a contact and press delete and it will prompt if you're sure, focusing the delete option so you can hit enter quick. would be better than doing it on your iPhone.
If you have MacBook and you sync your contacts with iCloud.. at least the mac lets you swipe left to delete rather than entering each contact details and selecting delete. This would be the fasted method.