Really, all you need is a strong STAB or non STAB Fighting move to take it down.
Sawk: 252Atk Choice Band Mold Breaker Sawk (Neutral) Close Combat vs 252HP/252Def Miltank (+Def): 101% - 119% (398 - 470 HP). Guaranteed OHKO. Choice Band wrecks even Defensive variants. Choice Scarf variants wreck SDef variants easily. Adamant Choice Scarf OHKOs SDef variants. 252Atk Mold Breaker Sawk (+Atk) Close Combat vs 252HP/4Def Miltank (Neutral): 102% - 120% (402 - 474 HP). Guaranteed OHKO
Emboar: Choice Band wrecks Miltank. Defensive: 252Atk Choice Band Reckless Emboar (Neutral) Superpower vs 252HP/252Def Sap Sipper Miltank (Neutral): 109% - 129% (432 - 510 HP). Guaranteed OHKO. Specially Defensive: 252Atk Choice Band Reckless Emboar (Neutral) Superpower vs 252HP/4Def Sap Sipper Miltank (Neutral): 137% - 161% (540 - 636 HP). Guaranteed OHKO.
Gurdurr: Gurdurr 2HKOs at +1 with Drain Punch, while Miltank cannot do much. Defensive: 252Atk +1 Guts Gurdurr (+Atk) Drain Punch vs 252HP/252Def Sap Sipper Miltank (Neutral): 67% - 79% (266 - 314 HP). Guaranteed 2HKO. Specially Defensive: 252Atk +1 Guts Gurdurr (+Atk) Drain Punch vs 252HP/4Def Sap Sipper Miltank (Neutral): 84% - 99% (332 - 392 HP). Guaranteed 2HKO.
Absol (Superpower): Defensive: 252Atk Absol (+Atk) Superpower vs 252HP/252Def Sap Sipper Miltank (Neutral): 55% - 65% (220 - 260 HP). Guaranteed 2HKO. Specially Defensive: 252Atk Absol (+Atk) Superpower vs 252HP/4Def Sap Sipper Miltank (Neutral): 69% - 82% (274 - 324 HP). Guaranteed 2HKO.
Also, Bulky Ghost types such as Misdreavus counter Miltank really easily. Miltank hates Taunt, and easily walls Miltank's Body Slam and Seismic Toss with immunties. If it has Scrappy, Misdreavus will take care of that with a carefully timed Will-O-Wisp.
I'll edit as I find.