Regular Landorus's strongest possible attack while still outspeeding Mega Khan is a 252 Atk Choice Band Sand Force Earthquake, which manages to secure the OHKO after one hit of Stealth Rock or two turns of sandstorm damage.
252 Atk Choice Band Sand Force Landorus Earthquake vs. 4 HP / 0 Def Mega Kangaskhan in Sand: 309-364 (87.7 - 103.4%)
However, standard Landorus use Life Orb, which, in addition to requiring both SR and two turns of Sandstorm to guarantee the OHKO, means that if Mega Khan survives, its Return is a guaranteed OHKO after SR and LO recoil, and a little better than even money to OHKO after just LO recoil.
252 Atk Life Orb Sand Force Landorus Earthquake vs. 4 HP / 0 Def Mega Kangaskhan in Sand: 269-317 (76.4 - 90%)
252 Atk Parental Bond Mega Kangaskhan Return vs. 0 HP / 4 Def Landorus: 265-312 (83 - 97.8%)
So you're correct in that having Mega Khan eat a SR with sand support allows Landorus to win, but without sandstorm on the field, Landorus loses, and loses heavily.