This is a core that you can use on balance and bulky offense. The sets don't really matter here; it's the synergy between the members that really makes this core what it is. Reuniclus has been a great win condition for a long time; it absorbs Scalds and can help cover the Fighting weakness shared by the other members of this core. Reuniclus is notorious for being able to set up on a number of common walls and offensive 'mons alike, and it can often bring wins even when the other members of the team don't. Cobalion mainly covers Reuniclus's Dark and Bug-type weaknesses, and it switches in easily to a majority of the cell's checks. It can also use Volt Switch to bring Reuniclus in at the right time, Stealth Rock to wear down common threats, and even Thunder Wave to give Reuniclus more setup opportunities. Cobalion can also provide a team with a secondary win condition with its Swords Dance set. Both Reuniclus and Cobalion are checked easily by most Ghost-types, which is where Hydreigon comes in. Not only is this 'mon always a strong wallbreaking presence, but it provides a second stallbreaker next to Reuniclus, which can disrupt an opponent's momentum. It can check some Electric-types for Cobalion as well.
This core mostly has problems with Fairy, Fighting, and Fire-type Pokemon. One great teammate is Tentacruel, which can beat all of these decently well and set Toxic Spikes, which benefit every member of this core.