You want a real Fairy Tail example for shit writing? Natsu vs Zancrow(keep Akainu vs Ace in your mind as I explain).
Natsu is a fre dragon slayer and can eat, along with being immune, to flames equal or weaker than his. Halfway into the series, he faces Zancrow who is a fire GOD slayer, and we learn that there are hierarchies in slayer magic. Since God slayers>Dragon slayer, Zancrow was immune to Natsu's flames and could burn him.
How does Natsu win? Very simple. He "emptied his own magic", ate Zancrow's flames, combined it with his dragon flames, and one shot Zancrow. 1) Where did "empty your magic come from? And how come Natsu didn't get hurt more? 2) How could Natsu eat god flames when it established a chapter before that he cant eat them after trying? 3) How tf did Zancrow get hurt by flames hes immune to?