Any tips on losing 12lbs in just under 3 weeks?

So before someone says "that's unhealthy, lose slower". I know that. Under any other circumstances that's what my goal would be. BUT. I joined this 6 week challenge where they give you a meal plan and make you workout 3x per week. They say you should reach your goal (mine is to lose 17lbs in 6 weeks) we're in the 3rd week now and I've only lost about 5lbs. They took a deposit of $400 which I will only get back if I meet my goal, so I need to lose it because I need my money!!

I feel like the diet plan set me up for failure as most of the people in the challenge are much more overweight than I am, and since they have more to lose and the diet plan (varies from 1400-2000 Cal's) causes them a higher deficit in calories it works better. I'm 5'8" and was 173lbs at the start of this challenge. I did the math and with being on 1400 cal per day and working out 90min of HIIT per week, it's just not causing enough deficit to lose 17lbs in 6 weeks.

So I need to crank it up like 10 notches to get my money back. I'm eating around 1200-1400 calories a day, I'm gonna start going to 4 of their sessions a week and try to do Zumba the other 3 days.

I figure if a few days before weigh in I'm still over I will take laxatives to remove waste and water weight.

Any other suggestions?