The easiest way might be just shift-pgDwn to mark them all

If you want to delete more than one email in one go.

If deleting eg: 6 emails in a block Click on first email, hold down 'Shift' key and click on last email. The block of six will be highlighted and display as 'conversations' in the Message pane. There will be a 'Delete' button.

if you want to delete various emails: hold down 'Ctrl' key and use mouse to click to select. All selected emails will be highlighted and display as 'conversations' in the Message pane. There will be a 'Delete' button.

Sometimes, I prefer to set a specific TAG on an email so I can easily locate later to select to delete.

I created a 'Kill' Tag. I then select the email and apply the Kill Tag to that email.

Then, later on, I either: sort by Tag, highlight the block and click on Delete. Or you could create a Message Filter which when manually run on folder does this: Tags contains Kill Perform this action: Delete Message