European X-ray Free Electron Laser (XFEL) in Hamburg will be most powerful FEL source in the world. It will deliver coherent X-ray pulses with femtosecond duration and high repetition rate up to 4.5 MHz. For beam conditioning all the end stations at European XFEL will be using focusing optics, the grazing incidence KB mirrors or/and Compound Refraction Lenses (CRLs). For precise design and specification of optical components, one faces problems that require full-scale wavefront simulations, taking into account optics imperfections and diffraction effects on apertures. We present application examples of wavefront simulations for various focusing optics schemes at the European XFEL, such as extreme focusing with long KB mirrors, concurrent impact of diffraction on the apertures of the mirrors cutting the beam and quality of the mirror surfaces. status of software framework for start to end simulation for single particle imaging experiments is also described.