I thought about it more and, in an environment that wants to balance offense and defense, I think I'll be using Garchomp, Gyarados and MAggron as a core. They cover each other's weaknesses perfectly. Garchomp covers Gyarados, who in turn covers MAggron, who in turn covers Garchomp. It's by no means perfect. Dragon+Fire with enough firepower can wreck it for example, but they all hit like a truck, minimizing safe opportunities to switch in. It's also weak to status, so it's best if Gyarados or MAggron takes the role of Reststalker. Alternatively, you can replace MAggron for Magnezone if you're strapped for the mega slot. It's just a tad boring.