The first ladder (512 x 128) of the DEPFET Sensor with Signal Compression (DSSC), under development for ultra-fast imaging of soft X-rays at the European XFEL, is instrumented and the key features have been qualified in a dedicated beam time. Pixel readout is based on a Depleted P-Channel Field Effect Transistor (DEPFET) with non-linear response designed to jointly provide low noise and high dynamic range. The XFEL beam hits an Aluminium (Al) target and the DEPFET ladder is mounted perpendicular to the XFEL beam to collect Al Ka fluorescence photons. Low-intensity X-ray spectra of Al Ka photons (< 1 ph/pulse/pixel) were acquired to study noise and gain versus integration time and temperature. The best obtained Equivalent Noise Charge is 8 electrons rms. Combined with a high gain value of 40 eV/ADU, we could achieve the impressive discrimination of the Al Ka peak with signal-to-noise ratio of 50:1.The qualification of the pixel non-linear response up to several MeV/pixel was obtained by increasing the beam intensity on the target with fine steps. Different techniques to precisely calibrate the pulse energy deposited into individual pixels have been investigated and will be discussed in detail. In particular, the linear range of the DSSC pixel response was extended more than a factor x1000 by covering selected regions of the detector with Al filters of increasing attenuation. A key improvement was the correction of the signal-dependent shift of the pedestal value, non-negligible at high illumination intensities, due to the power distribution scheme of individual ASIC columns. This was accomplished with a custom mask that allowed measurement of model parameters over the whole intensity range. The non-linear curves of all pixels have been collected and the major features of compression will be discussed in detail. These results validate the expected performance and the employed techniques for the final qualification of the 1 Mpixel DSSC camera.