Doing a Twitter search by "from:yourusername keyword" will list all your tweets with an specified keyword. Once the search page has loaded you can run this script to delete all the found tweets. It may fail if the computer doesn't load the button to delete before we attempt to click it.

tweet-delete.js

var tweets = document.getElementsByClassName("js-actionDelete");
for (i = 0; i < tweets.length; i++) {
	tweets[i].click();
	document.getElementsByClassName("EdgeButton EdgeButton--danger delete-action")[0].click();
}