I'm calling water nymph with trail skillz and uber slow.
Anyways, after a few practice games to see which champion I'm best at farming with, I like my Zyra at mid, about 96 cs on the 10' mark, it's much easier to get wraiths over the wall with her than Karthus (optimally I'd use AP Galio but I still got some trouble killing all of the caster creeps together, as well as getting the siege one). Now to get that on ranked games...