That shield is really not suitable for driving that type of motor. It's like putting petrol (gas) in a diesel engine. It will run for a while but not very well and it's going to get damaged.
Look for anything with an A4988 or DRV8825 chip in it. Those two chips are both very good at driving stepper motors of that size.
While you're waiting for the stepper drivers to arrive on the back of a camel (and a spare motor to replace the one you're going to burn up) can you increase the voltage of your power supply? That will increase the current.