在口语中,gonna这个词比be going to和will更地道.
“打败”常用beat来表达,如果你听过Michael Jackson的那首Beat it就明白了.
另外,如果你愿意突出“sooner or later”,也可以将其前置.
答案就成了 Sooner or later,I gonna beat him.
Extention:但是,如果是老师布置的作业,（以我对大多数中国英语老师的理解）,他们肯定用be going to 或者 will,然后再告诉你,gonna不是formal English.这是因为他们无知,只知道follow老旧的路子,不会叫你们一些地道的东西.
The writer is a A-Level student in England,the words above could only represent my personal view at the moment.