If you don't want to lose... then simply don't lose?
You literally contracted with an OP goddess that turns people into someone unbeatable with a spear, no matter how incompetent they are. So it is not as if you should have any issues trumping your competition. Even your sister allowed the contract on account of knowing how much it would empower you. Don't tell me you are
more incompetent than the famously incompetent yuno was?
Or what, are you saying that the goddess is a sham who is actually really weak, and provided no significant empowerment, unlike what we saw with yuno? That they conned you (and your sister) into wasting your once-in-your-life contract on someone worthless, giving them power, but condemning your life to failure in return?
Noone would ever be such an evil and malicious villain. ...wait, actually, is that not
exactly what happened?!? Oh shit!
Spoken like a true loser.
To be fair, they did gamble on the match. So they are true losers, who lost a bet. And has a vested interest in disqualifying it as a foul.