Although I do not battle competitively that does not stop me from making the attempt to have a badass Absol. I hunt for the Nature and Ability I want and even though it may take a bit of time and effort I don't like to go with anything else. It's just something I like to do, plus it's a lot of fun.
First thing I like to do is find the right Nature. Because of its high Attack Power I like to hit things hard and so I prefer to go with an Adamant Nature as it increases its Attack Power and decreases its Special Attack. If I'm having trouble finding an Absol with that Nature I'll also take Jolly (increases Speed and decreases Special Attack) or anything else that decreases its Special Attack, but only if I'm REALLY struggling with finding one that has an Adamant Nature. That's how picky I am about it. Usually it isn't that difficult though so I haven't really needed to go with anything else. If all else fails though and I'm feeling impatient I can always go with a neutral Nature.
Another thing to keep in mind as well is Characteristics. However, they aren't something I worry about too much just because I'm still pretty new to them, plus trying to get an Absol with the right Nature and Characteristic feels like more work than necessary as a casual player for that small extra bit of stats. If I get lucky with the right Characteristics, awesome. If not no big deal.
Next up is Ability. This I'm pretty picky about too. I don't like going with anything but Super Luck which increases the critical hit ratio of moves. Pressure isn't bad with its making moves consume more PP, and Justified isn't bad either with its increasing Attack Power if hit by a Dark-Type move, but honestly how often is that going to happen? Nope, Super Luck is where it's at for me. This probably wouldn't matter so much if you plan to make use of Mega Evolution but it's still something to keep in mind for those times you may be unable to use it (hey, it happens).
Other things to look into are breeding and Super Training. Breeding is great for getting those moves Absol wouldn't be able to learn on its own (personally there aren't any moves that appeal to me), and Super Training is awesome for putting those EV's (effort values) into the stats you want as wild Pokémon battles can be a little bit random on where those stats end up. Attack Power and Speed would be good stats to put those EV's into but if you're concerned about Absol's modest Defense and Special Defense it wouldn't hurt to put some points into those as well.
Last up is moves. In both Pokémon X and Pokémon Alpha Sapphire my Absol seems to know Night Slash, Psycho Cut, Strength, and False Swipe. Though False Swipe has no value to Pokémon battles it is awesome for capturing those low level Pokémon you would otherwise take out in one hit, plus it seems to be hard for me to find a Pokémon that can actually learn it (in Pokémon White none of my team can learn it; I need an Absol now!). If I plan to do real battles I'd likely swap it out for Swords Dance. Strength is there only for the purpose of moving things around, but in its own right it is a decent move. I'm not quite sure what I'd swap it out for anyway. Night Slash and Psycho Cut are just plain awesome and I like to use them in conjunction with Super Luck.