Needing your most powerful technique to win doesn't necessarly indicate the level of difficulty. It depends on the level of control you have on this technique
1- If it's a technique you can't pull of when you want but only when you're in a life and death situation for example(but without side effects on the body), but it allows you to win, it's a extreme difficulty fight
2- If it's a technique you can use when you wants, but that takes a drastical toll on your body to the point of leaving you close to death after using it, it's also a extreme difficulty win
3- If it's a technique that takes moderate toll on your body, it's a mid-high difficulty depending on how much toll it takes
4- If it's a technique taking no toll on your body(so you can spam without drawbacks), it's a low difficulty