dcae you must have misunderstood me; while I don't think Focus Blast is
necessary it is the best option in slot 3 for sure. There's also a few things you and
Jukain said which I don't agree with at all.
Can you actually make EPower / Psychic / Sludge Wave the mandatory combo of moves? Focus Blast has a shitty miss chance, and doesn't really...do that much (besides let you break through Skarm). There's merit to it on CM because +1 actually 2HKOes Chansey, but on the standard all-out attacker/SR set, there isn't much purpose to it. Meanwhile Sludge Wave provides really useful coverage in general, especially on stuff like the Latis on the switch-in. It's generally a very safe move to spam and has a ton of use in KOing Clefable as well. I really wouldn't forgo it unless I'm running CM.
Can you please not? I can't see how Sludge Wave's "utility" can even be compared to all the stuff Focus Blast does. Focus Blast might be horribly inaccurate but it is still a terrific coverage option for Landorus and is, in my opinion, superior to Sludge Wave in almost every way. Besides smashing Skarmory to bits Focus Blast also OHKOs Mega Tyranitar (way more important than you'd think considering that it only needs one Dragon Dance to beat many offensive teams), 2HKOs physically defensive Mandibuzz who just walls you with Sludge Wave, does way more than Earth Power to Ferrothorn (we're talking about an OHKO against physically defensive Ferrothorn which is getting a lot more common with the rise of threats such as Mega Gyarados and Mega Tyranitar) and deals a solid 62% minimum to physically defensive Rotom-W. While Sludge Wave and Psychic both 2HKO with SR up, you can't really guarantee it in a metagame full of Defog and other stuff to keep it off the field (Excadrill, fast Taunt). Not to mention that most good or even average players won't just switch their Rotom-W into Psychic like that. Focus Blast also 2HKOs Chansey once it falls below 64% which, quite honestly, isn't very hard to do and OHKOs Air Balloon Excadrill/Heatran/whatever (mostly Excadrill).
On the other hand, let's look at what Sludge Wave does: 2HKOs Lati@s (I don't know why this is important considering that you can't outspeed them and considering that Bisharp is definitely one of your best partners and ~guarantees the kill on both of them with Pursuit), "kills" Clefable (it's beyond me why you'd ever want to kill
boosted Clefable with Landorus seeing how Earth Power easily mops the floor with physically defensive Clefable anyway. Both Sludge Wave and Earth Power 2HKO unboosted Clef and Earth Power even 2HKOs most Clefable with a hit at +0 combined with a hit at +1 not to mention that there are better ways to kill Clefable anyway), kills Togekiss (lol), does 2% more to Charizard Y (still far from a guaranteed kill after SR). The only "notable" thing it does is OHKOing Non-Assault Vest Azumarill which is pretty big for most offensive teams since you easily survive the Aqua Jet and Azumarill causes headaches for those kind of teams often. You also do around 2% more to targets that are immune/resistant to Earth Power and neutral to both Psychic and Sludge Wave while hitting the incredibly uncommon Celebi and Tangrowth for massive damage... yeah that's about it.
Last slot should be Focus Blast / SR / U-turn IMO. Focus Blast is good for coverage, SR for obvious hazard, and U-turn bones stuff like the Latis as they switch in, and generally allows you to get good momentum. It's especially useful in Lando Keld Tar cores, which are becoming increasingly more common, enabling especially Keldeo, as well as Tar, to get in much more easily. It's generally a great move that needs to be slashed. Rock Slide doesn't really do anything besides hit Zard Y and incoming Talonflame, which Sludge Wave is more than fine against -- it can be useful, but not slash-worthy useful. It is worth a 'Moves' mention at best.
There's many reasons as to why U-turn is a lot worse on XY Landorus than on BW Landorus. First of all, its premier "counter" in BW (before U-turn became a thing), Celebi, is nowhere to be seen. Neither is Cresselia. Which leaves Latios and Latias. While Latias was quite common in BW, Latios has mostly taken its place in XY because it can actually do damage. There are some occasional Latias, but they still aren't defensive enough to switch into Landorus frequently. With Defog available, Latios also has pretty much no room for Roost to be fit on. That basically means the only thing that can take Landorus' moves that is somewhat dented by U-turn is your standard Latios with no recovery. Secondly, Stall teams often have the best Landorus answer that exists (Chansey) which also takes negligible damage from U-turn while causing you to take Life Orb and maybe even more SR damage once you come back in. When using Landorus against Stall in XY you're better off making strategic Double Switches to conserve your HP for chip damage from Pokemon such as Clefable and Venusaur when you go for the kill against them instead of blowing them on unnecessary moves that don't actually do any lasting damage. This is even more true if you run Sludge Wave over Focus Blast and can't touch Skarmory at all. Offensive teams in XY also mostly deal with you through Offensive switching (that is, they don't have anything to take your moves safely so they're forced to switch Garchomp or Keldeo or even Thundurus into your Earth Power to force you out; worst case scenario, they try to sacrifice whatever is currently out while you U-turn into them and completely waste your turn and some of your health) since so few Pokemon actually want to switch into Landorus. Latios is the sole exception, but Latios is easier to remove than ever with the rise of Bisharp as a top OU Pokemon as well as the advent of Aegislash. Best case scenario, you U-turn into Latios and bring in something that beats it. Worst case scenario, you U-turn into their Garchomp for 15% while taking Life Orb, Rough Skin and Rocky Helmet damage while missing up on the opportunity to put a massive dent into Garchomp.
Rock Slide is also not as bad as you make it out to be. In addition to Zard Y (OHKOing it is pretty big since most people will stay in expecting to tank a Psychic/Sludge Wave/whatever) and Talonflame (not sure why this is relevant but whatever) you 2HKO Specially Defensive Zapdos (who can just switch into you and Defog for free without Rock Slide; even Sludge Wave barely dents SDef Zapdos), Specially Defensive Mandibuzz (which completely walls you without Rock Slide and can also Defog, even taking less than 45% from Focus Blast) and does a respectable amount to Gyarados (exactly the same as Sludge Wave, actually, and thats with a -Attack nature). There's probably a few more situations I can't think of/can't be bothered to think of, but I think that's enough for now anyway. Rock Slide is situational, but those situations are probably a lot more common/important than the ones where U-turn actually helps you out.
I agree that Modest should be removed from the first set. You can seperate CM and RP, I just gave you an option of what to do if you want to keep it as a single "boosting sweeper" but I guess seperating them does make sense, especially since they play really differently. While Rock Polish isn't the best set, it's certainly usable and very powerful against offensive teams since it's immune to Thunder Wave from Thundurus and faster than every Choice Scarf user. I think it still deserves a set.
Gravity is horrible, most of the things switching into you are, once again, offensive switches (Thundurus, Keldeo and Garchomp come to mind again, and you have the whole dilemma of the current Pokemon staying in and putting a huge dent into you like with U-turn). It doesn't even help against Stall so I honestly don't see why you would ever want to use it ?_?.
Hidden Power Ice is also horrible; Dragonite has dropped in usage a lot in XY and Landorus-T and Gliscor aren't as common as they used to be. Landorus-T also rarely runs HP Ice nowadays so it's not like it can do anything notable (I guess it can Knock Off) back to cripple you. I'd even leave it out of OO because it's just that bad.
The current slashes and set orders are fine in my opinion. Seperating CM and RP is fine I guess, but theres no reason to change anything else really.
Sorry for the length of this post but I felt like I had to post my thoughts on this!!
and sorry for any grammatical/spelling errors i made this is kinda long so i guess it happens!!