Yes I got that, but didn’t have an exact answer, so came up with the coupon idea – quick, free and built-in.
There is no way to bulk adjust selected prices in the core WooCommerce plugin so far as I’m aware. For variable products, there are bulk action price adjustment options which work on all the variations of that product, but I understand you are looking at doing so for lots of products, so no good.
There are some plugins, but it would be best to learn how they work on a dev site before letting them loose on your main site. There’s:
https://en-gb.www.ads-software.com/plugins/excel-like-price-change-for-woocommerce-and-wp-e-commerce-light/
and
https://www.ads-software.com/plugins/smart-manager-for-wp-e-commerce/
which give a spreadsheet like environment to enable you to set sale prices quickly on selected items. The free versions tend to be “lite” so really just to try before you go premium.
Then there is the excellent plugin: Booster:
https://en-gb.www.ads-software.com/plugins/woocommerce-jetpack/
This has a “Bulk Price Converter” module in the “Prices and Currency” section. The free version of the plugin can alter all prices at one go. Perhaps you could do so and manually edit-back the prices for the dozen products not included. You would need the premium version to be able to alter prices at category level. The premium version is not a lot of money and has loads of modules for other problems.
It should be possible to export all products as a .csv, edit in Excel and upload. I can see that going wrong a few times before it goes right, so again, best to try on a dev site first.
Checkout plugins at www.ads-software.com, woocommerce.com and codecanyon.com. I’d expect you would have to pay to get what you want. Sorry, I can’t recommend a particular plugin.