Grapploct is pretty much completely eclipsed by other Fighting-types, such as uh... well, name a Fighting-type. It's stats aren't
awful, it has good Attack, but it's also got average bulk and terrible speed, and there's a ton of other Fighting-types who do pretty much everything it can do but better. In a way, it's a lot like Magmortar in this regard. It's not terrible on paper, but when you consider how heavily outclassed it is, it is in practice.
It does get a couple interesting moves. The first is Topsy-Turvy, a fairly rare move that flips the target's stat changes, turning boosts into drops. A very potent move, if he lives long enough to use it on a dangerous set-up sweeper. Which he won't. It could be useful with a Focus Sash, but at that point you might as well just run a Pokemon with Metal Burst or Counter/Mirror Coat or something. Or Roar. Or any other method of dealing with set-up sweepers.
The next, exclusive to Grapploct and its pre-evolution, is Octolock. This move prevents the target from switching and lowers its Defense and Special Defense by 1 stage each turn. If you must use Grapploct for some reason, this is the move I would abuse. It's not hard to play around this move if you know what it does (such as immediately switching into something that can force Grapploct out... which is a lot.) but frankly Grapploct is so rarely used that many opponents won't take Octolock into consideration, potentially letting you trap and remove key Pokemon like Toxapex, Ferrothorn, and Brave Bird-less Skarmory.
Fair warning; this set is 100% a theorycraft. I've never used Grapploct competitively and likely won't use it outside of randbats or something, but if I was forced to use it, this is what I'd try first.
Grapploct @ Leftovers
Adamant - Limber
248 HP, 252 Attack, 8 Def
-Drain Punch
-Stomping Tantrum
-Octolock
-Substitute
The combination of Octolock and Substitute gives you the chance to trap certain defensive Pokemon (Toxapex is a prime target here, being the defensive backbone of many teams, generally not fearing Fighting-types, and completely susceptible to this set) and whittle down their defenses, eventually letting you KO them. Drain Punch is a decent STAB move that lets you recover some of the damage you'll take in the process, either from attacks or from Substitute. Stomping Tantrum is mainly for Toxapex as it otherwise gets largely redundant coverage with his STAB, but honestly trapping Toxapex is about the most useful thing you can do with this Pokemon anyways. Ice Punch, Sucker Punch, and Brutal Swing (with Technician as his ability) are coverage moves that he could theoretically make use of, but it's hard to justify dropping anything on this set as to do so would be to throw away what tiny little niche Grapploct has over other Fighting-types, and there really aren't many opponents he can hit with them that don't outspeed and OHKO him.