Heres an offensive one:
Klefki @ Sitrus Berry
Prankster with 196 HP / 108 Def / 4 SpA / 196 SpD / 4 Spd
Bold Nature
Iron Defense
Calm Mind
Draining Kiss
Substitute
Convert your klefki into a sweepsih pokemon. 4 Spd to outspeed other klefkis and set up a safeguard, Defensive evs can be changed, and 4 SpA because there was no other use of this one point in a level 50 scene. Iron Defense / Calm Mind as you want to, boos your defenses and offenses at the same time with calm mind, iron defense to give you the OOMPH in defense. Draining kiss for sustain, and substitute to avoid status.
Rather than using two boosting moves to boost all of Klefki's best stats, I feel like you could get much more mileage by moving all EVs into HP and Def, with the remaining 4 into Sp.Atk, and replacing Iron Defense with Flash Cannon.
Klefki@Leftovers/Big Root
Bold Nature
Prankster
252 HP/252 Def/4 Sp.Atk
-Substitute
-Calm Mind
-Draining Kiss
-Flash Cannon/Psychic/Hidden Power Ground
Klefki's strongest stat is Defense, so pump it up as much as possible so that Klefki can reliably take a hit the first few times. The item of choice is the reliable Leftovers or the seldom seen Big Root. Big Root increases the amount of HP restored from draining moves, not their power, as all draining moves do not recover the same amount of HP as the amount of damage dealt. Draining Kiss will be your primary offensive move, and Draining Kiss already recovers 75% of your HP, with Big Root bumping that up to 100%. And because Prankster means you get off your Substitute and Calm Mind boosts lightning fast, you can reach optimal power in just a couple of turns. The fourth move is up to you. Flash Cannon is good Steel STAB to wreck Rock and Ice types as well as opposing Fairies, but Psychic lets you hit Poison types whether they're grounded or airborne. HP Ground aims for the two types you don't hit effectively with Draining Kiss, being Steel and Poison, but they'll whiff completely against the airborne variants.
This Klefki works very well against Special Attackers because of Prankster, as Klefki is fast enough to get a Substitute to absorb a hit, as well as Calm Mind to get the Sp.Def boost to absorb whatever attack they use. Prankster Subs means practically immunity to status, and Draining Kiss will just restore all the damage you lost to enemy attacks or to the consecutive Subs.
Let's not forget to mention two of Klefki's unique moves, Fairy Lock and Crafty Shield.
Fairy Lock works better as a suicide move, since it prevents all pokemon on the field from switching out the next turn. There are two applications for this. Holding an Escape Button, Klefki uses Fairy Lock, eats a hit, and all of a sudden you have a guaranteed switch-in opportunity and the opponent will be unable to escape the next turn. There's a reason I called it suicidal, though, as Klefki needs to take a hit for it to work. On the other hand, if Klefki is doomed to faint, Fairy Lock ensures that your next switch-in will be facing the opponent without fear of switching out because of the move. There are loads of pokemon that can benefit from this, all of them requiring the right opponent to be locked in. It will either give you a huge set up opportunity or the chance to eliminate a threat from your opponent's team.
Crafty Shield isn't worth a damn in Singles, but fares way better in Doubles. It protects all pokemon on your side of the field from Status moves but not from damaging moves, so what's the point? Like I said, it fares way better in Doubles as all the status moves that you can block with Protect, Crafty Shield blocks. Did I mention it can be used consecutively without fail unlike Protect. It's worth noting that Wide Guard does not block status inducing moves like Dark Void, which Crafty Shield does block. All of a sudden, you want Klefki for your VGC team, don't you?
Oh yeah, and for the countering Klefki section, make sure to mention that Quick Guard will block Prankster boosted moves, which is very valuable in Doubles, and can be used consecutively like Crafty Shield.