I pretty much use whatever I want. I prefer to have some degree of type coverage of course, but other than that I don't put too much thought into it. I mean, in my latest run I use low-tier (when it comes to Smogon tiers anyway) mons like Dodrio, Kingler and Passimian. Reborn is challenging, but not the point where you're forced to run certain mons, cores or strategies to stand a chance.