Wrong. An exploit in gaming is an action taken by the player to circumvent mechanics or to achieve a goal which is otherwise impossible.

Examples of an exploits: Freezing Twintania, Teleporting anywhere you like instantly by telling the server false coordinates, Double-casting with BLM via a certain rotation bug, Duping items.

What you are referring to as an exploit is either:
1. Working as intended
2. Unbalanced

There is no prerequisite bug needed or out of the ordinary behavior required by the player to achieve the end result. If the fight is easier than intended, then it is unbalanced. If, for instance, you discovered that drinking a potion of wind resistance completely nullified the damage of the enrage, then capitalizing on this action would be considered an exploit.