There is no such built-in functionality for the current setup, and adding it would mean a major overhaul of the site, which isn't going to happen. It's also... hardly necessary. As others have suggested, you open the asset pages in a browser window, go through with ctrl+tab and click download on each one, then when you cirle back the page has reloaded and you click the download link, and go through all again, and when circled back, just click "download" on whatever page you're on - if MEGA use the sync-client, and voila. Even with 100 assets it takes maybe 5 minutes.