Complex sentence: The fierce resistance of the Song forces resulted in the Mongols having to fight the most difficult war in all of their conquests, and the Mongols required every advantage they could gain and "every military artifice known at that time" in order to win.
Split into simpler sentences: The fierce resistance of the Song forces resulted in the Mongols having to fight the most difficult war in all of their conquests. The Chinese offered the fiercest resistance of among all the Mongols fought, the Mongols required every single advantage they could gain and "every military artifice known at that time" in order to win.