Eh...to unlock Challenge Mode requires stupid and complex requirements that are unanimously hated (long story, look it up). But if you don't mind using a cheating device you can skip that, or if you're playing on a ROM you'd have to use a cheat code which you can most likely just look up. And yeah, Audino makes leveling up in Gen V much easier and more convenient, which is good.
As I mentioned, BW only has Drilbur, Tympole and Sandile until you reach Icirrus City when you can get Stunfisk, then Golett after beating Brycen. B2W2 has Sandile, Drilbur, Onix, Trapinch, Sandshrew, Baltoy, Numel (W2 only), Gligar, Piloswine and Golurk, in that order. For BW you'll only really have the past gen ones for the new Elite Four and Alder, and Cynthia and Morimoto.