Genesect was dealt with primarily by forcing it to switch into moves (eg. Latias DM) and using that and rocks to wear it down. I'm actually not sure I saw any scarfers that weren't Genesect AND weren't weak to Ice Shard... the damn bug is so popular it's usurped the role on tons of teams - I only saw Scarf Mence, Landorus(-T), Rotom-W and Haxorus. The former two dislike Ice Shard intensely, and neither can break Landorus solo either, apart from a predictive HP Ice (I would generally use Toed for Landorus-I). Haxorus also loses to Landorus, although I would often prefer Landorus->Politoed so I could conserve the former's HP. Generally I would have to play around Rotom - Latias doesn't really care about its moves which is nice, so once I got a Scald burn or Toxic on it I could stall it out pretty easily. Having 2 Protects is very helpful for this, although I sometimes wished I had it on Landorus too haha.
If I had fought a Scarf Terrakion, I would probably have relied on the combination of Landorus, Tentacruel, Latias and Politoed to beat it, depending on how much health they variously had left. Any other set is less threatening because Jolteon can easily revenge it provided rain is up. Genesect would probably have been a lot more dangerous to the team if I didn't have Jolteon - Latias isn't always enough to kill it and Ebelt coverage moves would shred through the rest of the team. I believe Lapras can tank a +1 Thunderbolt if Rocks are not up, although Waterfall does not OHKO. Tentacruel is probably in the same boat, although it is pretty rare for it to be at 100% because I make use of its resistances a lot. I definately agree with the sentiment that Landorus has to be played smartly, because losing intimidate before getting some solid damage on enemy sweepers is a big setback.
The main threat to the team was Latios, in my experience. Without a Steel, I had to Poison/Burn it and then variously switch around absorbing Meteors with Tentacruel and to a lesser extent Lapras. It would end up doing a lot of damage (think 1-2 kills) every time I fought it, despite Jolteon being an excellent answer once it was down to about 65%. I think pretty much every game I lost was to either Latios or a team with both Hippowdon and Tyranitar - inclusing one incredibly frustrating game in which a Latios survived Signal Beam with 2% of its health and proceeded to, I kid you not, sweep me with CM Luster Purge :(.