I think Earth power/HP ice/Focus blast/ uturn or rock polish is probably the ideal set. I don't see what you need psychic for. You aren't killing anything more with psychic than you are with HP ice + earth power... HP ice already kills breloom and you have the added benefit of nailing lando-t, defensive chomp, dragonite and the rare gliscor late game. Also earth power gets tenta and toxicroak.
Psychic OHKO Gengar, has a huge chance of 2HKOing Gyarados after Stealth Rock, guarantee the OHKO on Keldeo (which can survive an Earth Power at full health - this means 100% health because if Landorus runs a Modest nature it can OHKO Keldeo after Stealth Rock damage with Earth Power). Oh, and it also hits Rotom-W accurately, but Psychic can only 2HKO non-specially defensive versions of Rotom-W; those will still need Focus Blast to be 2HKOed. Lastly, Psychic OHKOes Conkeldurr 100% of time; Earth Power only 2HKOes.
However Landorus really needs Rock Polish to sweep or U-Turn to defeat its usual counters, and cannot spare a moveslot for Psychic; unless that moveslot is HP Ice, but make sure that you really do not have problems with Gliscor, opposing Landorus, and Dragon-types immune to Earth Power, before making this choice. Even so, I would not use anything besides the standard moves, so I do not see much reason to use Psychic at all.
Oh, and I do not know why people are not speaking of Modest Landorus. It has a much higher increased chance of OHKOing and 2HKOing things. For example, it is guaranteed to 2HKO Gengar with HP Ice after Stealth Rock damage, while Timid variants have + or - 15% chance of not doing so and they never 2HKO without Stealth Rock damage. Also, Modest Landorus has 43,75% chance of OHKOing Keldeo with Earth Power after Stealth Rock damage, and 6,25% chance of doing so without, meaning that Keldeo will really need to be at full health if it dreams having a decent chance of checking Landorus, and even then, it has a very small chance of failing in doing so. These feats cannot be achieved with a Timid nature, however.
Modest Landorus hits harder in general and while it has less speed, I think it is very viable over Timid Landorus for the reasons above. It only really misses out outspeeding a few things before a Rock Polish, and can still outspeed everything Timid Landorus can outspeed after a Rock Polish. However, a Modest nature should only be used with Rock Polish; U-Turn variants need Timid to compensate for their lack of a way to boost speed.
That said, there was a guy here that said the Sand Force Landorus has much less counters than the Sheer Force set. This is true; in fact, even those few counters can be bypassed by Gravity. However, Sheer Force Landorus requires much less suport in general and it hits harder than Sand Force Landorus if sandstorm is not active. It also do not receive Life Orb recoil for its main STAB and coverage moves. It is the Sheer Force set that makes Landorus so much broken.
