I have 1 skypiercer out of ~6 horn breaks. Which is about what is expected (15% drop). I want the ura rubies so I can sleep bomb with a bowgun, rather than my crappy morpheus knife.
Most runs we did we broke both horns after two bombs (one run, one of the SnS'ers got stuck in Ala's legs). But then again this was with people who weren't ********, so it was 8 LBB+ with bomb boost each time. I think a horn breaks every second stagger, so you need 2800 damage to break both horns, which is 16 LBB+ with bombardier.
Everyone should bring mats for 12 LBB+ (unless you have pyro), as well as sleep S and sleepyfish (including blademasters).
If you want tails, you can use 2-3 gunners, 0-1 morpheus knife, and 1 GS (jho is probably the best). Sleep ala, wake it with the GS on its tail, and a gunner should deto before Ala's head is out of the way. You lose the double damage on one bomb, but do a crapload to Ala's tail, and you should get more damage overall. Wake it 4 times with a fully charged slice on its tail and the tail should come off. If you don't break the horns in 2 bombs, then you can finish them off with crag S/wyvernfire/bounce bomb+. Alternatively someone with tenebra D might be able to take it off as well, especially if you're parabombing once or twice (tenebra user just focuses the tail rather than bombs).
On a separate note, slicing shot is really weak. I wouldn't bother trying to take off a tail solo with a bowgun.