I can't imagine it'd be too hard to catch, as long as you take a pokemon with Skill Swap along with you on your horde adventures. I've been trying to get all the horde ones and haven't encountered major problems yet, besides rarity of some (and Geodude knowing Magnitude >.>). --- 3DS FC: 4055-4159-3260; PM if you want me to add you for Friend Safari goodness! Mine is fairy with Dedenne, Jigglypuff, and Floette (red flower).
Well first you have to catch a Gogoat and a Roserade. Then you need to find the alter dedicated to Arceus and take the Gogoat and Roserade there. Sacrifice the Gogoat at the alter and take the Roserade's petals and spread them around the alter and sacrificed Gogoat. If done right Arceus should accept the sacrifice and allow you to encounter one next battle. On a serious note the trading boards may be quicker, and you may even get a breeding leftover with some 31s --- Midnight Knights Co-Leader and Bow/Arm Master Gotta Stay Upbeat Upbeat Upbeat! Or You'll Be Dead Meat Dead Meat Dead Meat! FC 0533-4504-8392