Before going to Agate, remember to catch every available ground type mon, since a lot of them have useful niches (such as Hippo setting sand or Claydol with Trick Room). Also use the strength glitch to get the life orb in the icy rock cave, so you can abuse it with Nidoking and remember to get a Smooth Rock to use with Hippowdon (for the Sand Rush fun). Also, get all the store stickers because some of them become unavailable after returning to Reborn.
The easiest battle was by far Luna, if not for the field effect that prevented me from spamming earthquake I would have easily swept her with my moxie Krookodile (still it killed 5 of her mons). Aya, Cal, the Team Meteor battles, Noel, Radomus and Shelly were also very easy.
The hardest ones were a few of the Fern battles (but after he got rid of that unholy Ludicolo he got much easier), Serra (mostly because I didn't want to break her field because I didn't want to lose relationship points with her), and Samson (because 2/3 of my team was weak to fighting and my only mon that had STAB against him was Claydol). I still haven't gotten to the Blake battle yet, but I do have a solid plan to defeat him and don't see myself having much trouble against him.
However, the battle I'm truly afraid of is Amaria, she is literally the bane of a mono ground run. The only thing I can think of to have a small chance of defeating her is Trick Room + Choice Band + Torterra + Wood Hammer spam, hopefully that works...