Sableye may only be found in Reflection Cave, at a 5% chance.
"Jack of all trades, master of none" is the phrase best suited for Sableye. While it may have a good variety of attacks in its arsenal (albeit mostly by TM) they're marred by its lacking stats (the highest are a mere 75). Likewise, while Dark/Ghost is a great defensive type they're rather redundant as far as offensive coverage goes. Sableye's not a gimmick, but it's definitely more of a "fun" Pokemon than a "strong" one.
Rival (Tower of Mastery): Meowstic is doable; Shadow Claw/Payback is a 2HKO, while it would have to crit with both Disarming Voices to defeat Sableye. Absol is too much for Sableye. As for the starters, Payback works well on Braixen and Frogadier but Quilladin is just a bit too bulky.
Gym #3 - Korrina (Shalour City, Fighting-type): Lead with Hone Claws and proceed to sweep through with Aerial Ace. Machoke's the only one who can even touch Sableye and only with Rock Tomb, though for safety's sake you'll still want the power boost.
Rival (Coumarine City): Same team, same strategy; wreck Meowstic, Braixen and Frogadier with Payback and avoid Absol and Quilladin.
Gym #4 - Ramos (Coumarine City, Grass-type): Jumpluff stomps Sableye with Acrobatics, unfortunately. Weepinbell's damage output is just sad, being a 4HKO at best with Grass Knot; bombard it with Aerial Ace if faster or Payback if slower. Power-Up Punch followed by Paybacks will serve well against Gogoat, though Payback spam alone is highly likely (albeit not certain) to work as well.
Aliana (Kalos Power Plant): With Intimidate nerfing its already meh-ish Attack and the presence of Swagger antics, Sableye would have to be rather lucky to pull off a win here.
Gym #5 - Clemont (Lumiose City, Electric-type): A pair of Paybacks will polish off Emolga, though with Volt Switch it's unlikely that Sableye will be able to pull it off unless Emolga's the only one left. Avoid Magneton and Heliolisk for the same reason; their Thunderbolt is a 2HKO, they both require at least two hits to take down, and they outspeed Sableye to boot.
Rival (Route 14): Meowstic is still Payback fodder and Absol should still be avoided. Even with the type advantage Delphox is not safe; Payback's a 2HKO while there's a small chance of it 2HKOing with Mystical Fire. Success against Greninja is largely dependent on how lucky it gets with Water Shuriken - honestly it's safer to just use something that can easily take Water moves. Chesnaught's no good either; its Seed Bomb is just too strong.
Gym #6 - Valerie (Laverre City, Fairy-type): Mawile only has Dark moves and Iron Defense, so burn it with Will-O-Wisp and Dig, Dig, Dig to victory. (The burn will be doing most of the damage; Dig's mainly there to reduce the number of hits Sableye takes.) Afterward, plunk Sableye on the bench, as it has no business fighting 'mons with STAB Dazzling Gleam.
Celosia and Bryony (Poké Ball Factory, tag battle with rival): Power-Up Punch the Liepard until it dies and then Dig the Manectric. It's a bit dependent on how much the AI gangs up on Sableye but unless you're very unlucky it should manage just fine.
Mable (Frost Cavern): Dig twice and win - it's a 2HKO and while Houndoom's Foul Play may be as well it's a very slim chance and easily negated by holding Leftovers (or by Houndoom not using Foul Play twice.)
Rival (Anistar City): The only thing here Sableye can take on is Meowstic (with Payback, naturally). Everything else is too fast and hard-hitting for Sableye's poor stats.
Gym #7 - Olympia (Anistar City, Psychic-type): The much-faster than Sableye Sigilyph can score a 2HKO with Air Slash and Sableye can't one-shot it without a lucky crit. Keen Eye Sableye can try for the 2HKO with Shadow Claw against Slowking but should bail out if it survives two hits; Stall Sableye get a guaranteed 2HKO with Payback, though. Regardless, beware of Calm Mind and Yawn antics and know that Sableye will fail if Reflect's active. At least her Meowstic is as easy a target as any other you've fought (once again assuming Reflect is not up).
Lysandre battle #1 (Lysandre Labs): Apply Zen Headbutt to Mienfoo until it dies and then get Sableye out of there.
Aliana (Lysandre Labs): Sableye really has no business being here unless it's to soak up Intimidate.
Celosia (Lysandre Labs): Sableye doesn't have power to cut down either Manectric or Drapion before they do the same to it.
Bryony (Lysandre Labs): Slap on Dazzling Gleam and proceed to ruin Liepard's day - even with STAB behind them Sableye can take those Sucker Punches. Avoid Bisharp; Iron Head's a clear 2HKO which is more than Sableye can manage in return.
Mable (Lysandre Labs): Neither Houndoom nor Weavile is a safe fight for Sableye.
Xerosic (Lysandre Labs): Crobat's too strong, but Malamar's a potential target; provided Sableye is slower Malamar's Payback is only a 4HKO while Dazzling Gleam knocks it out in three turns.
Lysandre battle #2 (Team Flare Secret HQ): Mienfoo's now a too-strong Mienshao and the others are as unbattleable as ever.
Xerneas (Team Flare Secret HQ, X only): A legendary Fairy type? Pass.
Yveltal (Team Flare Secret HQ, Y only): More reasonably typed than its X counterpart, but still way too mighty for Sableye.
Lysandre battle #3 (Team Flare Secret HQ): Sableye couldn't handle Lysandre before and he's even stronger now. Avoid.
Professor Sycamore (Couriway Town): Sableye doesn't have enough power or durability for any of Sycamore's Pokemon.
Shauna (Route 19): If Sableye has been taught Brick Break, it might be able to take out Greninja depending on how many times it hits with Water Shuriken. Everything else is off-limits.
Tierno (Route 19): There's nothing here that Sableye's low stats can handle.
Trevor (Route 19): See Tierno.
Gym #8 - Wulfric (Snowbelle City, Ice-type): Abomasnow is straight-up off-limits for Sableye. Avalugg is doable by using Will-O-Wisp followed by Power Gems, though if Will-O-Wisp misses Sableye should be pulled out.
Rival (Victory Road): Like before, Meowstic is the only safe target.
Elite Four Wikstrom (Pokémon League, Ironworks Chamber, Steel-type): Klefki wrecks Sableye with Dazzling Gleam. Probopass can be brought down by spamming Dig or Brick Break; non-Stall Sableye can also make do with Low Sweep or Power-Up Punch. Scizor's too hard a hitter, taking off over half Sableye's health with a single X-Scissor or Iron Head. Sableye might be able to beat Aegislash if it can land a Will-O-Wisp straight away and avoids Iron Head flinches but it's far safer to just burn-and-retreat.
Elite Four Malva (Pokémon League, Blazing Chamber, Fire-type): If Sableye still has Power Gem it can take out Torkoal, otherwise avoid it. Everyone else is too hot for Sableye to handle.
Elite Four Drasna (Pokémon League, Dragonmark Chamber, Dragon-type): Dragalge is only beatable if Sableye scores at least one Zen Headbutt flinch and doesn't miss; a risky proposal. Druddigon could theoretically be beaten with Dazzling Gleam spam if it weren't for the fact that the only move it can use against Sableye is Dragon Tail. (it does make for a good Will-O-Wisp opportunity though.) Altaria's Moonblast spells doom for Sableye. Noivern is a no-go; every one of its moves (apart from Super Fang of course) will knock out Sableye first.
Elite Four Siebold (Pokémon League, Flood Chamber, Water-type): Everyone of Siebold's Pokemon is too strong, too bulky or both for the likes of Sableye.
Champion Diantha (Pokémon League, Radiant Chamber): Burn Hawlucha with Will-O-Wisp and then Zen Headbutt to victory. Tyrantrum's Head Smash crushes Sableye, with a high chance of one-shotting. Brick Break 2HKOs Aurorus but Aurorus's Blizzard (and possibly Thunder, though that's not guaranteed) is also a 2HKO; battle only if Sableye's faster or if Aurorus either leads with a screen or misses. Gourgeist is another Will-O-Wisp target after which a Payback or two will finish the job, though note that Sableye can't take more than one non-burned Phantom Force so beware of missing. Goodra's too darn bulky and Gardevoir has Moonblast.
Post-Game: Sableye's post-game success is mixed at best and largely only wins out against the earlier postgame by virtue of being higher leveled. Still, congrats for bringing it this far!
A freshly caught Sableye will have Astonish, Fury Swipes, Fake Out and Detect; nothing too impressive, though Fake Out has its uses. Shadow Sneak at level 25 helps it get around its low Speed but is too weak in most instances. Ignore Knock Off, Feint Attack and Punishment at levels 29, 32 and 36 as Payback is usually stronger and the effects of those move are nitchey. Shadow Claw at level 39 would be nice if you didn't already have the TM for it. Level 43's Power Gem is more useful than you'd think; Sableye's Special Attack isn't much lower than its Attack and there are a good handful of Rock-weak enemies. Level 46's Confuse Ray is risky. Foul Play at level 50 is a potential replacement for Payback, if only because it runs off the opponent's Attack stat, which is more often than not higher than Sableye's. Level 53's Zen Headbutt should have already been picked up from the relearner by now. Shadow Ball at level 57 is already available by TM, but could be inferior to Shadow Claw (depending on IVs/EVs/etc.). The last level-up move is Mean Look at level 60; even if it were useful, it could've been relearned by now.
Sableye has a good mix of TM moves. Power-Up Punch, Low Sweep and Brick Break are all good for coverage and can be switched out in anticipation of upcoming battles. Payback is pretty much a staple, with Sableye's low speed granting the boost in most situations (or all of them, if Sableye's a Stall specimen). Will-O-Wisp can help Sableye or its teammates deal with high-Attack 'mons. Other miscellaneous coverage moves to be rotated as needed include Dig, Dazzling Gleam and Aerial Ace.
Steel-types: Not only do they counter Sableye's worst nightmare (Fairy-types), but they are typically high in Defense, something that Sableye lacks. In return, Sableye can no-sell any Fighting moves that would threaten its metallic comrade.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Which ability do I want? Oh, if only Prankster weren't a hidden ability! Guess you'll have to settle for Keen Eye; it's not terribly useful, but at least it's not an outright hinderance like Stall.
What Nature do I want? It can't do much with its Base 50 Speed, so a Speed decreasing nature is best. Note, though, that Quiet natures will take longer to reap the benefits as it takes a while to start acquiring special attacks.
How good is Sableye in a Nuzlocke? Interesting to play around with but too lacking statwise for serious gameplay.
Immunities: Normal, Fighting, Psychic
Neutralities: Flying, Ground, Rock, Bug, Ghost, Steel, Fire, Water, Grass, Electric, Ice, Dragon, Dark