40k Titan air defense questions

[aurak_merc]

Hi all,

I have a 40k question for those who may have the sagely answers and advice that I seek. To begin, I have never played even 1 game of 5th edition 40k, and only played 2 small games of 4th edition. I am mostly just a collector and hobbyist as I tend to stink at most wargames and have almost no time to play them. Models are what drag me in.

So, on to the situtation: I am building an Apocalypse army. In that army is a Reaver titan (armorcast, not FW). I have heard that flyers tend to chew up titans with relative ease, and defending against them involves only a couple of options.
1.) Get weapons with AA notations on them
2.) Get a Hydra Flak tank
3.) Put a Vulcan Mega Bolter on the Carapace of the Reaver (read this on some website a while back, but forget where)

I understand points 1 and 2, as they are essentially the same. Other than pintle mounted weapons, I am not aware of any other Imperial weaponry that is "AA". If there is, what might those be?

I do not get point 3 though. Is it just because the megabolter throws out so many dice that you are playing the odds of getting 6's? If that is the case, wouldn't a Stormlord tank be just as good if not better with the ability to fire twice?

Finally, I will have a group of 3 Valkyries in the army. At what altitude are they considered to be "flying"? Would they block the Reaver line of sight to its potential targets? Would they block attack lanes from enemy flyers just by hanging out around the Reaver?

Sorry for the (most likely) noob questions. If it matters, the only flyers I count on seeing are Orks, with a very slight chance of Imperials.