Kingler achieves perfect coverage with Return + Crabhammer, with the exception of Shedinja. Therefore, you can use an SD + Agility set and sweep with remarkable ease. Gardevoir makes a very good supporter for this strategy, as it can set up screens, has Lunar Dance/Memento (does Xatu?), and can also pass Wish.
With the hype around the MilKing core, talk has been about wallbreakers that can smash through both. However, I've been trying Gligar recently and it actually has better defensive synergy with Slowking. Gligar has two weaknesses - Water and Ice - and Slowking resists both. Both are also predominantly Special, so Slowking does not have to worry about tanking strong physical attacks, as it must when paired with Miltank. The Electric immunity is very nice, as fast, LO/Specs Electric-type moves were tearing through my team - Manectric was a huge problem. Miltank doesn't cover this weakness.
One of the bigger advantages of Gligar, though, is its access to U-Turn - Miltank would often have to fire off blind Body Slams at switch-ins and hope for paralysis. This ruins offensive momentum and can let strong special attackers wreak havoc. U-Turn allows you to bring in a poke that can benefit from the opponent's switch-in, hopefully letting you bag a kill or at least turning the tide of battle in your favour.
Thoughts?