I just figured out a beastly moveset for Slowking! Won me 2 Online Battles in a row without switching out or losing any Pokemon! I'll post the codes for the Battle Videos in question at the bottom. =D
Slowing (M) @ Metronome <------Key to the whole Learnset!
Trait: Regenerator/Own Tempo (Doesn't matter to much for this particular learnset as the trick with Metronome is to keep on swingin'.)
Sassy Nature (+SpDef, -Spd)
- Trick Room
- Nasty Plot
Psychic and Surf are your STAB moves. Both of these moves have 90 Base SpAttack and 100% Accuracy AND they both cover for each other. If you know what you're doing then with these two moves alone, you'd be hard-pressed to find yourself without coverage. (Unless you find yourself fighting a Grass/Steel type or another Water/Psychic)
Using Trick Room as an opening move, combined with having your Speed lowered by Sassy Nature means you're almost always going to have the edge on Speed as well as other Speed Boosting/Lowering Moves.
The Sassy Nature will make your Slowking nice and bulky and as such, it goes masterfully with Metronome/Nasty Plot! After Trick Room is casted, use Nasty Plot to power up Slowking (If necessary), and then proceed to attack with your STABS.
Here is the key part: As I'm sure you know, Metronome powers up any given move every turn as long as the same move is used in succession. (Repeatedly). Now; Take Slowking's high Special Attack and coverage, combine it with power from the Metronome and Nasty plot, and you get a combo that'll KO nearly any opponent in two hits minimum, if not one! =D
Battle Video Demonstrations:
Code #1: 576W-WWWW-WWW5-D298
Code #2: KTYG-WWWW-WWW5-D28R
(Tip: To view Battle Videos in X/Y: First; Connect to the internet. Then go to your "Key items" pocket, select your "Vs. Recorder", and then hit "Search By Code") =D