The multiwavelength anomalous diffraction (MAD) method is used to determine 
phase information in x-ray crystallography by employing anomalous scattering 
from heavy atoms. X-ray free-electron lasers (FELs) show promise for revealing 
the structure of single molecules or nanocrystals, but the phase problem remains 
largely unsolved. Because of the ultrabrightness of x-ray FEL, samples 
experience severe electronic radiation damage, especially to heavy atoms, which 
hinders direct implementation of MAD with x-ray FELs. Here, we propose a 
generalized version of MAD phasing at high x-ray intensity. We demonstrate the 
existence of a Karle-Hendrickson-type equation in the high-intensity regime and 
calculate relevant coefficients with detailed electronic damage dynamics of 
heavy atoms. The present method offers a potential for ab initio structural 
determination in femtosecond x-ray nanocrystallography.