This situation can be tricky. I usually delete them one by one - click an element, press delete, repeat. But in your case that might take a long time.
It's likely that the elements are numbered in two groups of consecutive numbers. In that case, if you can identify the lowest and highest number of each group, you can use Edit -> Select elements by number and enter a range like 123-456 to select all elements from number 123 up to number 456.
Comments
It's likely that the elements are numbered in two groups of consecutive numbers. In that case, if you can identify the lowest and highest number of each group, you can use Edit -> Select elements by number and enter a range like 123-456 to select all elements from number 123 up to number 456.