It's possible to do it, just difficult. On my first run in story mode we did it on accident and still downed it (we thought that was what was supposed to happen). Tried it once on HM for larfs but was way to hard on the healers. For guilds just starting Kephess I agree with the OP that this fight was not intended to kill the walker in two phases.
You killed the big walker just in 2 phases on Hardmode?
You must have imba dps xD