Much like the guide above, I also recommend using Darkseid's Omega Beam (Bk,Bk,X).
TIPS FOR FINAL BATTLE: DARK KHAN
For most people out of all the battles, this is the one that they unfortunally lose to, but their still is a tactic even though you spam a move. When he counter attacks you, do not stress as when you get up, immediantly do the Omega Beam as the beam connects unlike Kanos which goes only half-way across the screen.
Once the ending for Darkseid comes up, the achievement should unlock.