I've used Landorus T a few times, and have come up against him more than a few times, so here's my thoughts on the Pokemon you could use.
Cloyster @ Focus Sash
Ability: Skill Link
EVs: 252 Atk / 4 SpA / 252 Spe
Jolly Nature
- Shell Smash
- Icicle Spear
- Rock Blast
- Ice Shard
Take a hit, set up Shell Smash, following move take Landorus T out. Disadvantage here is that you'll need to take a turn to setup, so that Landorus T may be forced to switch out. Advantage here is that you now have Cloyster setup to wreck havoc to the rest of his team, or just switch him out.
Mamoswine @ Choice Scarf
Ability: Thick Fat
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Icicle Crash
- Earthquake
- Rock Slide
- Superpower
Choice Scarf won't outspeed Landrous T, and will lock you in to a move, and Landorus T may have switched out by then. But you could second guess and select another move to anticipate the switch out. An alternative to this set is to choose Choice Band and go for Ice Shard. With it's +1 priority, STAB and Choice Band boost, you have a 81% chance of OHKO Landrous T.
The real problem with the sets above is that they are physical attacks and Landorus T has Intimidate, to they either have to switch in and survive potentially two attacks, which is asking a lot without defense investment, or take the -1 to attack and try their luck. The damage calcs show that only Cloyster can guarantee a OHKO post-intimidate. But Mamoswine with Icicle Crash still has a 95% chance of OHKO post-intimidate.
And thinking out loud, (I've not used this before so cannot really comment on it's usefulness), Gligar resists all attacks, can learn Ice Punch and Knock Off. Ice Punch is guaranteed 2HKO, and Gligar has Hyper Cutter as an ability, so it is resistent to Intimidate. Maybe a set worth trying for Gligar is below (I don't know if you are playing on Pokemon Showdown or are playing Battle Spot, so this stuff is always easier said than done!)
Gligar @ Eviolite
Ability: Hyper Cutter
EVs: 248 HP / 252 Def / 8 SpD
Impish Nature
- Roost
- Earthquake/Ice Punch
- Swords Dance
- Knock Off/Ice Punch
With +2 Attack, Ice Punch becomes a OHKO on Landorus. The most damage Landorus can do is 19% with Knock Off, so you can easily switch into Landorus when it is locked in to another move (Eviolite users are crippled by Knock Off), set up Swords Dance, and do tons of damage. Roost is useful late game.
A worthy mention should also go to Gligar's bigger brother, Gliscor:
Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 244 HP / 40 Def / 224 Spe
Impish Nature
- Substitute
- Toxic
- Protect
- Earthquake/Ice Punch
The best Landorus can do is a 5HKO. Use Protect, activate Poison Heal so now Gliscor is immune to Knock Off. No one likes being poisoned, so use Toxic to phase him or just go for broke and use Ice Punch (but Ice Punch will only be a 3HKO even after poison damage, again Intimidate is a factor.)
Last but not least, consider Mega Banette:
Banette @ Banettite
Ability: Frisk
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Protect
- Will-O-Wisp
- Knock Off
- Destiny Bond
Use Protect whilst mega evolving, then you can use Knock Off. Banette will survive whatever attack Landorus T is using, even Knock Off. Then with Prankster use either Destiny Bond or Will O Wisp. It's risky, but it could work.
