Let's set things straight.
A Giant-Killer Einherjar can never beat a Mountain Giant, Giant Killer or not, with boost or not, even if he attacks the Mountain Giant first before the the Einherjar is attacked. I tried it personally in the editor. It's called "Giant-Killer" probably only as based on Mythology (they probably for for the Aesir gods against the giants, but I'm not sure if I'm correct), but this is a game. A classical age unit can't beat a heroic age myth unit.
A Hill Giant is a Mountain Giant, an Ice Giant is a Frost Giant, and a Lava Giant is a Fire Giant. They're all the same.
Oh, and the Frost Giant breathes extremely cold mist enough to freeze humans. Jormund Elvers breathe venom, not steam. Norse Mythology never depicted them breathing steam (personally I think Jormund Elvers are even fictional, since Jormugand never had children). In reality, Fire Giants would probably melt the Frost Giants down, but I don't think the Frost Giants were even MADE OF ICE, rather, they blew cold mist (remember, frost giants blew mist, not ice, ice is a solid, not a gas) which actually confuses me, because frost giants don't look like they're made of ice, but if you attack them the hitting would sound like you were hitting buildings, which made clear evidence that the Frost Giant was indeed, made of hard ice. And the Fire Giant's skin was, according to the AOM dictionary, made of boiling lava. Freezing the lava would turn it into rock, thus freezing the Fire Giant, which I believe would be somehow realistic. By the way, Mountain Giants are not made of rock, rather they inhabit mountainous areas.
This reminds me of crazy myth unit vs. myth unit encounters where the stronger one actually dies just because of retarded abilities (such as Minotaurs beating Cyclopses just because they can gore, Medusae turning Colossi into stone, Frost Giants freezing Fire Giants, and Mummies turning Phoenixes into Minions and ending up without any egg).
[This message has been edited by Budada (edited 04-18-2010 @ 11:00 AM).]