Common digestive and lung disorders such as stomach problems and pleural emission pose serious difficulties for patients who undergo open surgeries wherein a wirelessly powered swimming microrobot has the potential for minimally invasive missions from diagnosis to drug delivery and surgery. However, remote steering toward an arbitrary destination is yet a challenge to existing microrobots. Here, a resonant wireless power transfer system is incorporated into an array of planar coils to generate spatio-temporal patterns of magnetic fields on an untethered swimming microrobot. As shown, this generates an effective Lorentz force on the microrobot's power receptor coil. Automated switching of the array current creates a traveling magnetic pattern that enables smooth microrobot steering and controls its velocity. This eliminates the need for embedded magnets and shielding materials that suffer from nonlinear characteristics. The microrobot's function is validated by an in vitro experiment that mimics the anatomy of the stomach filled with fluid at a centimeter range underneath the skin. Our microrobot with a mass of 7 g is able to reach the velocity of 0.45 mm/s in a controlled direction by applying a small peak magnetic field of 2.15 mT at 96 kHz. The propelling force is proportional to the square of the total magnetic field's intensity and can be enormously increased depending on the application. Compared to microrobots with similar dimensions, this method results in a 50-fold increased ratio of propelling force to applied magnetic energy, which is suitable for carrying heavier payloads to farther locations.