What I did to do these as alliance was log on my horde toon and i defeated the first few trainers . Up to and including Cassandra Kaboom. I stopped for a bit and noticed the quest shares over to my alliance so while i was over in kalimdor I finished off what i had left.
Do I need to do all the battle masters on my horde alt to get this achievement or is it just bugged?
Edit: I relogged and even instance zoned to see if that would correct it, it did not.
Edit#2: I tried logging on my horde to see if it would trigger completion. still nothing