I got a burnin' question that I can't answer on my own....
Say you have an Aegis wall. On one side of the wall is some Scarabs. On the other side is some Wave Serpents. How far away to the scarabs and/or Serpents have to be from the wall in order for the scarabs to be completely hidden behind the wall? I don't have any Wave serpents to test it out.
BTW, this is the missing piece of an equation... the other 2 pieces are...
Q: How many Scarabs does it take to usually kill a wave serpent on the charge?
A: 2.
Q: Assuming the aid of a Tomb Spyder, how close do the scarabs have to be in order to usually charge a wave serpent? (regardless of terrain)?
A: 24".
OK, scarabs will kill a wave serpent in combat. But so will anything that can reach them, it's just the usual AV 10 back armor thing. Problem with scarabs is that you have a lot of STr 6+ that ignores cover. A wave serpent could make you pick up the entire unit in one shot. They have no special reason to be within 24" of you.
Matt is right, the easiest way to kill them is CC, unfortunately not all armies have CC and wave serpents are tough to catch flying all over the damn board.
So what are some good shooting options? well they're AV12, there is a lot of Str7 floating around in Tau missiles and Autocannons which is still only a 30% chance for a glance or pen, Str8 with Marine missile launchers, Psyriflemen, Melta (which is going to give you a better chance anyway if you're close enough)
and of course there are tons more Str7,8,9,10 weapons out there. The problem with the higher stuff like Rail Cannons, Manticores, Orbital bombardments, etc is you're probably going to only get 1 hit on a Wave Serpent and even if you get an explosion the Eldar player may just turn it into a glance.
The most reliable way of shooting them to death seem to be glancing the hell out of them, i've tried firing 10-12 twin linked IG autocannon shots at one and still not getting it done. Hopefully i'll have better luck with Tau missilesides