Onix can be played whenever you want, but Aggron V/VMAX and melmetal are both outstanding attackers in this deck. Try to avoid using onix's attack too much and use switches to get him in and out. Use nest balls and ultra balls to find melmetals, meltans, onix, aggron V/VMAX. magnezone greatly assists the energy requirements for aggron and melmetal.
Play this deck and feel free to comment or make your own adjustments.