We have developed an experimental technique to investigate a nonlinear magnetization dynamics resonantly excited by strong microwave fields. In this technique, a magnetization switching was observed when a sub-nanosecond wide pulsed field was applied to the ferromagnetic resonance-excited ferromagnets. Such a cooperative switching with simultaneous application of microwave- and pulsed-fields can evaluate an effective energy barrier for magnetization switching in various nonlinear magnetization dynamics. In this review, we show the experimental results on (1) the temporal evolution of the energy barrier in the transient of magnetization precession growth and (2) the observation of saddle-node bifurcation in the microwave-excited nonlinear magnetization dynamics.