Meltabombs hit at Initiative 1 because they have the unwieldy rule.
No, TH/SS termies won't do anything to it. Str D ignores invo saves, so it's no different than with normal termies, and I think Stomp does too. They don't have nearly enough attacks at Str 8 to kill it, they might take a few hull points off is all.
Wraith knights would do better than any other MC, but still would still be unlikely to kill it, on average. The imperial knight kill the Wraith knight pretty easily, though. I do not think there is any MC can really kill it before it promptly dies.
There's no real way to catch it with EMP grenade carrying FW, or really anything on the eldar side that has to come out of an enclosed transport. SO while haywire's are a good answer I think the only unit that really can do it is wyches out of a raider. Everything else will die first. That goes for sister repentia, too. It would be great if you could put them in a LR but you can't. (also, the knight with the melta blast can kill LRs pretty easy)
No, every army does not have an answer to superheavies.
Rob, 2 hammerheads are cheaper than an imperial knight, but what does that matter? I'm saying even the hammerheads are plenty hard to kill, the knight will be even tougher, and it doesn't get stunned or immobilized. I said 18 or 36 because you can take 3 or 6 of them.
Super-heavies are a bad concept outside of apocalypse. 3-6 of them is absolutely ridiculous. Just because GW wants to sell everybody Imperial Knights is not a good reason to wreck the game over it. (more than it already is)