Try this:
Step 1: Turn off sync & backup on the google photos app on your iPhone. This will cause any action taken in the web browser to NOT be synced to your phone - which then will allow photos to be deleted from the web browser without that action being reflected on your device because it's no longer synced.
Step 2: Access your photos through google photos in a web browser. Here you will find all your photos that have been synced and stored in the google cloud. You can delete the photos in the web browser and they will not be deleted from your device because the device is no longer 'synced'. You can select multiple photos at once by selecting one, holding shift, scrolling down to the bottom of the page and selecting another one.
Note: Once you turn off sync, you cannot delete the photos directly from the google photos app on your phone. Even though sync has been turned off, this will still delete the photo from your phone. You must turn off sync and then delete the photos through a web browser.
After doing the above test with deleting just one photo and make sure it's still in iCloud. Once you confirm it didn't delete from iCloud, proceed with deleting the rest.
Good Luck