Miltank - Item: Leftovers. EP Spread: 252 HP. 252 Def. 4 SpD. Nature: Impish (+Def, -SpA) Ability: Scrappy (for those hard-to-reach Ghost types)
Milk Drink - Miltank is a fantastic wall. Nothings is more frustrating than bringing her down only to be healed right back up.
Toxic - Does a great job with stalling
Counter - If you predict a strong attack, Miltank will most likely survive and destroy your opponents Pokemon. (I've survived Close Combat from a Heracross with 88% HP left...with it's attack doubled. Heracross was not so lucky) This will also cover Miltank's inability to use toxic if the opponent is either a Ground or Steel type. I'm very surprised that this move wasn't in any of the other answers. It has worked wonders for me.
Seismic Toss - Whether you're dealing with a special attacker or fighting a Pokemon who isn't greatly effected by counter, this is your only option of attack. More than likely, you can pull off Toxic and take them down piece by piece.
I do not recommend using Curse. Miltank has a very decent speed for a wall Pokémon. The speed may come in handy if you need to use toxic, seismic toss, or if you need to heal in a pinch.
The only weakness I see in this set is if I get hit with Toxic or Will-O-Wisp, it tends to slow me down slightly. Be sure to keep a Pokemon with heal bell or aromatherapy just as an insurance policy.
Don't be afraid to put Miltank as your 1st or second Pokemon. This set is made to put a great dent in your opponents team. I hope this helps.