Roggenrola for Low. It comes a lot later than in B/W1 and at that point imo isn't really worth the effort, it's a lot more "meh" in B/W2, and you'd think it'd be able to train against Burghs gym, but the abundance of Sewaddle line mons make it VERY difficult to do so. Being slow as molasses and having an AWFUL offensive movepool (Earthquake's postgame now so it runs on mono-Rock essentially unless you tutor it Iron Head and Superpower, the former gives you pretty much zero coverage and the latter isn't until Yamaji, oh and did I mention you need to have Gigalith to even be able to LEARN THESE MOVES) also hurts it considerably. I'd say Low for Gigalith access, Bottom for Boldore, as Boldore can't even learn Iron Head and Superpower ._.
Roggenrola also has the Dig TM as an option, which you conveniently remembered for Heracross and Pinsir, yet for some reason left out here.