User blog comment:Miskos3/Fairy Tail 528: Dragon of Magic, Review/@comment-24403746-20170325195920/@comment-7418318-20170326080109

Wendy wasn't all that (magically) exhausted, but she was severely injured.

The better question to me isn't how she can go into Dragon Force, but how she can stand upright without bleeding out. She can't heal herself after all and Erza can't heal her either.