I'm a real experienced user of Marowak, and this is a set which I feel works great.
Marowak @ Thick Club
152 HP/252 Atk/104 SpDef
Brick Break/Fire Punch
It's all on personal preference really, a Brick Break OHKO's no HP, no Def Hydreigon. Fire Punch is usually for type coverage. Bonmerang is to break sash users, and to deal non-slashed damage, as well as not damaging a partner, but that can be taken care of if you pair Marowak up correctly. Earthquake's advantage is that it's a spread move, and it never misses, unless stats are altered. Rock Slide, you have. It's ev spread is a little more complex, I find that those extra special defense ev's work quite well, in allowing it to stay alive, so I recommend changing that, and it's ability should be either Lightningrod, or Battle Armor, in case one of your Pokemon has Thunderbolt, or a single target electric move.