hi, i found out an easy way to make your encounter rate with rare pokemon.
1. put a pokemon of the same of type of the rare pokemon you are looking for.
2. if you don't have a pokemon of the same type of the rare pokemon you are looking for, put a rare pokemon that you have caught earlier. (for example, if your looking for plusle, and you don't have a minun yet, use ralts/kirlia/gardevoir as the top pokemon because ralts is also encountered rare.
3. the most important of all, patience... yeah thats crucial.
I've experienced this because I caught my elusive volbeat when I put shedinja in the first slot of my party pokemon due to the fact theyre both bug type. another one is that I also encountered minun (common encounter) and after that plusle(rare) when I had kirlia on top of my party pokemon. It also works when I was looking for Skitty, Chimecho and Grimer. As for Grimer, I caught a gulpin and put it on top of my party pokemon. (I had a hard time looking for grimer than Chimecho hehehe) so yeah, hope it helps.
PS I don't use cheats or whatsoever. PEACE!