typically, investing in HP is better than investing in Def/Sp.Def, since it increases your bulk versus everything, not just one or the other. Bright Powder is also an unusual choice for volcarona; maybe a life orb or focus sash would be better. Dry Skin is an interesting choice; it makes you kind of weak to sun, which is not worth the benefits of anti-rain (since Water is only normally effective anyway.) Poison Touch might help you a bit more since you really only have Milotic to status people. Since Milotic is your status absorber, you might need less Def investment, since Marvel Scale provides an amazing defense boost.
As for your issues with tank/stall, setup counters them. Swapping Haxorus out for a different lead, and switching it in on the enemy cleric/tank.