My technique would be to use the Search tab in the browser. Select the tasks you want deleted, using the search criteria. Then use the Multi-edit feature, on the search results page, to update the Completed status of the tasks to Yes.
Like JPR said, I would back up my tasks first.
If they aren't searchable, I suppose you can add a Star or a Tag to them, and then search on that, again multi-editing them to 'completed' status.
Or you could save them, weed out the ones you don't want, delete all your ToodleDo tasks, and then reimport the smaller list.
The point is, there are several ways to do this, and which one is best probably depends on your situation. I'm sure someone will add another idea or two as well.