The Set:

Landorus-Therian @ Choice Specs  
Ability: Intimidate  
EVs: 252 HP / 252 SpA / 4 SpD  
Modest Nature  
IVs: 0 Atk  
- Hidden Power [Ice]  
- Grass Knot  
- Earth Power  
- Psychic / Focus Blast

What it beats:

Mamoswine 
     252+ SpA Choice Specs Landorus-Therian Grass Knot (120 BP) vs. 0 HP / 4 SpD Mamoswine: 556-656 (154 - 181.7%) -- guaranteed OHKO 
     (Scarf and Fast Band are able to beat it)

Torterra
     252+ SpA Choice Specs Landorus-Therian Hidden Power Ice vs. 4 HP / 0 SpD Torterra: 428-504 (128.9 - 151.8%) -- guaranteed OHKO

Zygarde
     252+ SpA Choice Specs Landorus-Therian Hidden Power Ice vs. 252 HP / 252+ SpD Zygarde: 276-328 (65.7 - 78%) -- guaranteed 2HKO
     252+ SpA Choice Specs Landorus-Therian Hidden Power Ice vs. 252 HP / 252+ SpD Zygarde-Complete: 276-328 (43.3 - 51.5%) -- guaranteed 3HKO

Swampert-Mega
     -1 252+ Atk Swampert-Mega Ice Punch vs. 252 HP / 0 Def Landorus-Therian: 292-348 (76.4 - 91%) -- guaranteed 2HKO 
     252+ SpA Choice Specs Landorus-Therian Grass Knot (100 BP) vs. 0 HP / 0 SpD Swampert-Mega: 568-672 (166.5 - 197%) -- guaranteed OHKO 
     (Fast Special Swampert-Mega can beat Landorus-Therian)

Steelix-Mega
     252+ SpA Choice Specs Landorus-Therian Earth Power vs. 252 HP / 220 SpD Steelix-Mega: 350-414 (98.8 - 116.9%) -- 93.8% chance to OHKO 
     -1 36+ Atk Steelix-Mega Heavy Slam (120 BP) vs. 252 HP / 0 Def Landorus-Therian: 129-153 (33.7 - 40%) -- guaranteed 3HKO 

Quagsire
     252+ SpA Choice Specs Landorus-Therian Grass Knot (80 BP) vs. 252 HP / 4 SpD Quagsire: 700-824 (177.6 - 209.1%) -- guaranteed OHKO 

Donphan
     -1 252+ Atk Donphan Ice Shard vs. 252 HP / 0 Def Landorus-Therian: 136-160 (35.6 - 41.8%) -- guaranteed 3HKO
     252+ SpA Choice Specs Landorus-Therian Psychic vs. 0 HP / 4 SpD Donphan: 209-246 (65.1 - 76.6%) -- guaranteed 2HKO

Gastrodon
      252+ SpA Choice Specs Landorus-Therian Grass Knot (60 BP) vs. 252 HP / 4 SpD Gastrodon: 436-516 (102.3 - 121.1%) -- guaranteed OHKO 

Garchomp
     252+ SpA Choice Specs Landorus-Therian Hidden Power Ice vs. 4 HP / 0 SpD Garchomp: 428-504 (119.5 - 140.7%) -- guaranteed OHKO 
     -1 252 Atk Garchomp Outrage vs. 252 HP / 0 Def Landorus-Therian: 144-169 (37.6 - 44.2%) -- guaranteed 3HKO 
     (252+ Specs Garchomp with Hidden Power [Ice] will win.

Diggersby
     -1 252+ Atk Huge Power Diggersby Breakneck Blitz (200 BP) vs. 252 HP / 0 Def Landorus-Therian: 306-361 (80.1 - 94.5%) -- guaranteed 2HKO 
     252+ SpA Choice Specs Landorus-Therian Focus Blast vs. 0 HP / 0 SpD Diggersby: 460-542 (147.9 - 174.2%) -- guaranteed OHKO

Excadrill
     252+ SpA Choice Specs Landorus-Therian Earth Power vs. 4 HP / 0 SpD Excadrill: 594-698 (164 - 192.8%) -- guaranteed OHKO
     -1 252+ Atk Excadrill Corkscrew Crash (160 BP) vs. 252 HP / 0 Def Landorus-Therian: 216-255 (56.5 - 66.7%) -- guaranteed 2HKO

Flygon
     252+ SpA Choice Specs Landorus-Therian Hidden Power Ice vs. 0 HP / 4 SpD Flygon: 444-524 (147.5 - 174%) -- guaranteed OHKO
     252+ SpA Flygon Hidden Power Ice vs. 252 HP / 4 SpD Landorus-Therian: 248-296 (64.9 - 77.4%) -- guaranteed 2HKO

Garchomp-Mega
     252+ SpA Choice Specs Landorus-Therian Hidden Power Ice vs. 4 HP / 0 SpD Garchomp-Mega: 388-460 (108.3 - 128.4%) -- guaranteed OHKO
     252+ SpA Garchomp-Mega Hidden Power Ice vs. 252 HP / 4 SpD Landorus-Therian: 328-388 (85.8 - 101.5%) -- 12.5% chance to OHKO

Golem
     -1 252+ Atk Golem Stone Edge vs. 252 HP / 0 Def Landorus-Therian: 124-147 (32.4 - 38.4%) -- 98.4% chance to 3HKO
     252+ SpA Choice Specs Landorus-Therian Earth Power vs. 0 HP / 0 SpD Golem: 594-698 (197.3 - 231.8%) -- guaranteed OHKO

Rhyperior
     252+ SpA Choice Specs Landorus-Therian Grass Knot (120 BP) vs. 248 HP / 244 SpD Solid Rock Rhyperior: 633-747 (146.1 - 172.5%) -- guaranteed OHKO 
     -1 252+ Atk Rhyperior Continental Crush (200 BP) vs. 252 HP / 0 Def Landorus-Therian: 276-325 (72.2 - 85%) -- guaranteed 2HKO

Gliscor
       252+ SpA Choice Specs Landorus-Therian Hidden Power Ice vs. 244 HP / 108 SpD Gliscor: 412-488 (117 - 138.6%) -- guaranteed OHKO 
       -1 252 Atk Gliscor Subzero Slammer (120 BP) vs. 252 HP / 0 Def Landorus-Therian: 308-364 (80.6 - 95.2%) -- guaranteed 2HKO 
       (Poison Heal)

What it loses to:

Landorus
     252+ SpA Life Orb Landorus Hidden Power Ice vs. 252 HP / 4 SpD Landorus-Therian: 411-489 (107.5 - 128%) -- guaranteed OHKO 

Seismitoad
      252+ SpA Choice Specs Landorus-Therian Grass Knot (80 BP) vs. 252 HP / 204 SpD Rindo Berry Seismitoad: 248-292 (59.9 - 70.5%) -- guaranteed 2HKO 
      52+ SpA Seismitoad Hydro Pump vs. 252 HP / 4 SpD Landorus-Therian in Rain: 434-512 (113.6 - 134%) -- guaranteed OHKO 

Swampert
       160 SpA Swampert Hydro Cannon vs. 252 HP / 4 SpD Landorus-Therian: 404-476 (105.7 - 124.6%) -- guaranteed OHKO 
       252+ SpA Choice Specs Landorus-Therian Grass Knot (80 BP) vs. 0 HP / 184+ SpD Rindo Berry Swampert: 204-240 (59.8 - 70.3%) -- guaranteed 2HKO 

Piloswine
       252+ SpA Choice Specs Landorus-Therian Focus Blast vs. 252 HP / 0 SpD Eviolite Piloswine: 374-440 (92.5 - 108.9%) -- 56.3% chance to OHKO
       -1 252+ Atk Piloswine Icicle Crash vs. 252 HP / 0 Def Landorus-Therian: 372-444 (97.3 - 116.2%) -- 87.5% chance to OHKO 
       (Basically a loss because focus blast misses)

Gliscor
       252+ SpA Choice Specs Landorus-Therian Hidden Power Ice vs. 244 HP / 108 SpD Gliscor: 412-488 (117 - 138.6%) -- guaranteed OHKO 
       252 Atk Gliscor Subzero Slammer (120 BP) vs. 252 HP / 0 Def Landorus-Therian: 460-544 (120.4 - 142.4%) -- guaranteed OHKO
       (Hyper Cutter)

Stunfisk
     84+ SpA Stunfisk Hidden Power Ice vs. 252 HP / 4 SpD Landorus-Therian: 212-252 (55.4 - 65.9%) -- guaranteed 2HKO 
     252+ SpA Choice Specs Landorus-Therian Earth Power vs. 252 HP / 172 SpD Stunfisk: 356-420 (84.3 - 99.5%) -- guaranteed 2HKO 
     (But it has to have a lot of special bulk)