Wordgirl has a pretty significant AP advantage, 42 kilotons vs 12.9 kilotons.
Her abilities aren't garbage either, but Tsuna has the advantage of probably being a bit more willing to kill right off the bat. Plus, the fact that he possesses Ice Manipulation probably makes Wordgirl's icy breath null and void. And that forcefield of his probably allows him to take a few more hits from Wordgirl while he battles his way around a 3.26x AP advantage.
In short, Tsuna is just too versatile and skilled for the flying grammar nazi
Didn't really see the need to put more given the reasoning already above mine but sure why not?
Tsuna has better versatility in combat, more skill (afaik), can mitigate her AP advantage with forcefields, clair and precog, flight makes her earth manip far less effective if not irrelevant, and also comes with a considerable range advantage to wear her down with his flames before she can even try to hit him.