Complex sentence: Once the fighter defeats the other warriors, he will open the "Ultra Vortek", combat the Guardian, and if victorious, may then use his newly acquired power to determine the fate of the Earth.
Split into simpler sentences: Once the fighter becomes victorious and defeats the other warriors, he/she will open the "Ultra Vortek" and combat the Guardian. Once he/she defeats the Guardian, he/she may then use his newly acquired power to determine the fate of the Earth.