Specially Defensive pokemon: If Blissey, Chansey, Zapdos, or some other good specially defensive wall is on the opposing team, be sure to keep Staraptor healthy to deal with it. Otherwise you may find you lose all your pokemon to a very slow Blissey sweep, which isn't a pleasant experience.
Pokemon boosting their speed: The team only has one (fairly fragile) scarf user and no priority, so if something boosts its speed it can suddenly be a threat. For example, if Cloyster sets up a shell smash then unless you can phaze it with skarmory it will sweep the team. Be certain to get up stealth rock against teams with cloyster as this will help defeat it before it can set up.
Stealth Rock: This team (as with many flying teams) struggles when Stealth Rock is up on its side of the field. The Volt-Turn core is hit hard by taking 25% with each switch (even with tornadus-t's regenerator), Zard takes 50% on switch-in and Skarmory loses Sturdy. If the opponent seems likely to go for Stealth Rock, do what you can to defog it as early as possible (even if it means losing your own hazards), as otherwise you'll have a very difficult time winning.
Electric teams: Ok there's two electric immunitites on the team, but many electric pokes also carry ice coverage so that's not too helpful. Moreover, there's plenty of flying or levitate pokes on most electric teams so Landorus will struggle to sweep. Often they have Zapdos as well, making things even harder. There's no simple solution here, just be very careful and try to preserve your pokes as best you can as generally it's quite easy to wear them down slowly, allowing for a landorus sweep when the ground immunities are on low enough health to be KO'd by Sludge Wave or Focus Miss. Being stally also helps break those annoying balloons.
Bulky Water teams: Yeah these are the best teams on the ladder right now in my opinion. If you can help it, don't battle
Sae Sae cuz you're in for a beating if you do. Thundurus-t can spam Volt Switch and MegaZard can spam Solar Beam (if they don't have Politoed), but the amount of BoltBeam on these teams isn't nice. Don't be afraid to sacrifice pokes here, because if you win it'll be with only Zard, Thundy and Torny surviving in any case (at best). Staraptor 2HKO's CroCune if Scald doesn't take it out. Most people don't expect it to do that much, so use that to your advantage and take the momentum. Other than that, uh... good luck. These teams are pretty much the best counter teams I can think of (better than electric, ice or rock).
Any user going by the name of Pk-Kaiser: Yeah, that guy's ungodly good at pokemon. Uh... hax him to death...
and probably still lose.