The whole idea of a Nuke is to do a “clean” Nuke, ie to wash the baby but not throw the baby out with the bathwater. In other words you are wanting to Nuke the STYLES back to Normal Style, but not remove Italics and Bolding etc.
Essentially that means that the advice to use a double copy/paste via a straight text editor (eg NotePad) is out. You need to use a RICH text editor (eg WordPad) if you are going that way.
A simpler way is to do a Select All in Word and apply Normal Style to the lot. However if this is a Word file from someone else, then THAT Normal Style could be a total mess and have “hidden” formatting (eg Borders) that could take some effort to remedy.
A far better way is to do the (single) copy/paste from that Word file to a BLANK (new) Word file, which (hopefully) already has your preferred settings for Normal Style as well as all the other styles we will discover below.
But when you do the paste it is essential that you nominate the TARGET Styles (ie Match Destination Formatting), otherwise it could end up a total mess. See the Screenshot below.