Give -up - the word you are looking for is "overcome" which means to beat someone, or vanquish, conquer - such -like words. But unfortunately I do not think that works in this context. It would be better as follows: "If I fight you I'll beat you" or "If I fight you I'll win." The second is probably better because it reflects on you rather than on them. :)

The statement is not clear. If you mean "to overcome" or "win against somebody", then I suggest the following statements: "If I will fight you, I will win against you." "If I will fight against you, I will win." The word "overcome" is not applicable in the context of "fight" (physical) but is applicable it is fighting against disease, or against emotional problems, etc.