Chromatus said that Deathwing would be more difficult and he said this when Deathwing was still alive. It was also a weakened Chromatus who had not yet reached his full strength and even so they could not kill him completely.
And Chromatus is stronger than base Deathwing. Basic Deathwing barely defeated Alexstrasza. He only became so strong at the end of the raid because N'zoth gave him even more strength.