Well, I never actually thought she lost to Lucy. She went fairly easy on Lucy and still had the upper hand. But Lucy isn't even on the level of a high-ranking Spriggan anyways.
I doubt Brandish could have killed Team Natsu on Caracol Island though. Gray, Natsu, and Erza were all casual on the island. If those 3 were at their full-power, Brandish would have lost almost immediately.
That was supposed to be a spy/infiltration mission. Gray, Natsu, and Erza had no intention of starting a war with Alvarez around that time, which is why they did little to stop Brandish. If they wanted to, they could defeat Brandish right then and there.
I agree Natsu was cut, but the difference was that he proceeded to two-shot Neinhart without feeling much pain. And Brandish would lose the moment Natsu entered FDKM.
I do understand that Brandish wasn't serious the entire arc, but I'm not going to extrapolate her feats because of that. She may not have had any intention to fight, but she sure hasn't shown anything on the caliber of a high-tiered mage.
If anything, her feats have said otherwise.