If you can trade from HGSS, take a Mareep Egg called Sparky. Sparky must be traded to your Pearl the moment you have access to trading. It can hatch anytime before Floroma Town. To get it accustomed to the Sinnoh nature diversity, it must solo 15 Sinnoh-only Pokemon (eg. Combee) before it may evolve. After evolving into a Flaffy, it may not learn any new electric moves until it has defeated 5 electric types that are 5 levels below or above its level at the time. Afterwards, solo all the gym trainers in Pastoria City except those that have water/ground types before you can evolve into Ampharos. After its evolution to Amparos, it must defeat any Drifloons or Drifblims (except Fantina's one) in Fantina's gym (which means you have to get all the qeustions wrong). Upon reaching the final gym, it must have an epic showdown with Volkner, but soloing him!
If you cannot get a cute little Mareep... (which I doubt since that thing is in your siggy XD)
Take a Burmy called LeafShedder. It must be female (which means it has to evolve into a Wormadam XD). Before evolution, it must at least defeat one rock type. After evolution (which is at Level 20), it can evolve anywhere depending on your choice, but they have different restrictions. There is also a general one that you must defeat 10 types of Pokemon whose Hidden Power you got (eg, if you got Hidden Power Grass, you fight ten grass type. Let's hope you don't get Hidden Power Fire, otherwises it's going to be mission impossibru soloing fire types within Wormadam-Grass). If you get the Bug/Grass Wormadam, it must defeat five flying types not lower than 5 levels below it before it can learn a non-Grass type move after Confusion. If you get Steel/Bug Wormadam, it must survive 10 moves without lowering the foes' stats and potions. The foe must also be no lower than 5 level than it's level. If you get Bug/Ground Wormadam, your move pool is so bad that I'll just give you a hit-Fissure-once when you learn it restriction.