Props to Ronni for going mythological on them and really taking a very simple solution here. Swordsmanship be damned, she can literally just drop the sword on her and be done with it. She comes down to get that confirmation, fluidly sliding down from 300 to 60 feet as she does to cop a squat and confirm her win. Then Ronni is casual and cool again, back to being the easygoing gal.
Britomart immediately realizes what she is dealing with, in her terms, and really she isn't far off. The title of titan suits Ronni because here she is a power unto herself, and she is a creature of legend in a world of legends. Thus her concern that Ronni can't really be stopped or opposed so she has to hope for the best now- Britomart has no other choice at this juncture.
Might well all the Urth be undone indeed.
For those to whom it matters- Ronni is 300 feet tall in panel 1, then 60 in panel 2. Makes Britomart one inch tall to scale in panel 1, six inches tall to scale in panel 2. Because I do lots of math for this strip.