Well, difficult is a relative term. It all depends on your skill level and your teams skill level.
For us it was difficult, but tons of fun.
And yes, we did exploit a rock in the water that the zombies can't seem to climb up on. I personally consider it a bug in the game because I read somewhere that the developers tried to make sure that the zombies could go anywhere that the survivors could go.
Yeah, the guy who orchestrated the plan new about the glitch and told us the tanks wouldn't bother us while on the rock, but that wasn't true at all. We had two tanks come after us, and they would throw rocks at us (and hit us occasionally).
The smokers would also come after us in the water. We eventually all equipped the hunters rifle so that we could shoot these guys when they are still far away.
No way can you do this with the AI bots, they will take damage for sure. Don't get me wrong, the AI is surprisingly good, but something like this achievement requires a careful plan and obviously there is no way to make a plan with the AI.
We had fun. Enough said.