I've had success (although you would need to put wheels on) by attaching wrench to pedal with the crank arm at 6 oclock and the wrench at 9 (or 3 depending on which side you're looking / loosening), and then applying bodyweight / stamping on wrench.
@Hovis started
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I've had success (although you would need to put wheels on) by attaching wrench to pedal with the crank arm at 6 oclock and the wrench at 9 (or 3 depending on which side you're looking / loosening), and then applying bodyweight / stamping on wrench.