Ok... assuming that "252 spd" stands for special defense...
The thing is, you say as if having a choice of Body Slam / Dragon Tail is a really shitty thing: it isn't. That's because otherwise Miltank can't choose between them! You discredit Lickilicky for being unable to deal with Substitute setup users, but so is your Miltank. Even more significant is the fact that your Miltank takes 45% minimum from Life Orb Hydro Pump from Ludicolo, which means a high likelyhood of 2HKO with SR up. And the same move from Samurott will cleanly 2HKO. Ludicolo, on the other hand, takes 40% maximum: this means that even with SR you can easily wall the prominent special attackers.
Using Miltank as a special wall is a bad idea in my opinion, especially when you lose to the very things you are attempting to counter. If we are to extend on your simile, it's like trying to use a special wall that cannot switch into neither Latios nor Rotom-W without a great chance of 2HKO in OU.
As with Natu, you can't prevent SR with it. Mainly because of its weakness to Rock-type moves, and it's a bad idea to switch onto Carracosta / Golem what have you. You have to play a stupid prediction game just to keep SR away. The SR weakness after it is set up makes it really hard to repeatedly switch in. It gives free turns for Skuntank / Musharna etc and most importantly is unable to check most Fighting-types like all psychics are required to.
The thing is Natu is really predictable, and no one will really use bouncable moves unless he / she is really really sure that they got a proper read on you.