Don't bother botting for an onyx, it's extremely crowded at caves right now, people have been sharing this spot too much, and have completely murdered it. Not to mention, I see bots there with the gold farming setup (D med, granite plate, d skirt), so I'm sure Gold4RS is in the process of fucking this spot up, just like they do with other decent spots (Yanille Druids, Blue Dragons, etc).
tl;dr - It'd be better to find another spot, as Tzhaars get worse by the day.