No doubt, the Elshimo Slab drops more frequently at the Gemini Falls region at the top right of the map.
I've always farmed these on and off, and it was only until I tried farming there, I got an Elshimo Slab no problem.
I wasn't counting, but I probably killed 20-30 before getting the drop. Nearly 10-16 spawn in that whole area, so I imagine you could get the drop in one or two clean sweeps.
Oh yeah, and mind you, I mean OUTSIDE the waterfall, not where you fight the monkeys for ZM4 ^^;;;