Actually in this context i agree with Go korea. 'good luck' doesn't make any sense here. In fact i don't think 'good luck' is a good translation for 'fighting'. The feeling is different. I played a lot of sports tournaments in Korea and the feeling is different from 'good luck' in a lot of contexts. 'do your best' would be better
Its a borrowed word, just translate it as is. Colloquialisms will confuse people trying to learn.
It's not really borrowed as it doesn't have the same meaning, didn't come from having had it, and even violates the grammaticality of the word if it were borrowed directly. It's Konglish like 핸드폰. I'm also not sure I get the point you want to make. It's a commonly used word in Korean like many of the others in this course.
This pronunciation sounds odd, more like "pie-ting" than sounding close to "fighting". Not sure if this is an issue but it's good for others to know how to pronounce correctly.