I'll be totally honest, being someone that barely touches fairy if at all, this core looked pretty tough to work with. However, as I was teambuilding, I think I came up with some brilliant ideas, and this team really grew on me. This is especially for the fact that this works without Klefki, which I think is a pretty cool feat it itself. Now, granted, that wasn't what I originally planned or intended, but, I gotta say, I'm stoked with the end result I came up with, and I hope yall do too!
Mawile took me a bit to come up with a good set, I know I definitely wanted it as a lead, taking advantage of its great defensive typing (with neutrality to Steel and immunity to Poison), and Intimidate (one thing Mawile has that Klefki doesn't). Here I gave it Metal Burst as a way to deal serious damage to incoming threats, doing double the damage to both Physical and Special Attackers alike. Super Fang is to quickly widdle down health on walls, dealing half of the opponents health at full. Knock Off gives it additional utility by, as the name implies, Knocking Off items. This also gives me a semi-decent check to Gengar. Toxic can also be used to further stall out HP.
Whimsicott I've always had a secret crush on when used on Fairy Teams, because of its pretty handy Grass coverage it provides (which is extremely handy against Ground and Water especially), and its a pretty interesting Prankster abuser. With access to a ton of support moves, like Encore, Stun Spore, Leech Seed, Tailwind, ect, its fun to play around with. Here, I decided to do something interesting--since I am not running Screens, I thought it would be a nice surprise to run Memento! This gives a great opportunity for Azumarill to set up, and either it or Diancie to shred teams to bits! The set from here is pretty standard, given its basically my Klefki replacement with priority paralysis.
I really wanted a Stallbreaker on this team, which most Fairy Teams often lack, so I did a hybrid of certain someone's set, and added Nasty Plot. This gives me a great general Steel check, especially to Bisharp and Scizor. It also has a great amount of bulk to abuse Nasty Plot to the fullest, and easily break any stall situations I may get into with merciless flinch hax! For the last slot, I preferred Roost to keep its health up for as long as possible, but if you're really nervous about Heatran (even though Diancie 2KO's it, and Azumarill OHKOs), Aura Sphere can be used, however I don't see much of a need.
Diancie was the Mega I decided on early on, granting me hard-hitting rock coverage, providing me with more speed, some type of hazard control, and a pretty good scout. The set's pretty basic, but I'm running HP Fire over Earth Power, because I feel I get a lot more out of it, being able to potentially surprise KO Scizor on switchin, a chance to OHKO Skarmory after Sturdy is broken, and be able to hit Grass mons, compared to just being able to hit Heatran (whose Balloon needs to be broken first anyhow).
Azumarill was a no brainer to have on this team, giving me an aditional Steel neutrality, and a very reliable Physical Attacker and Cleaner. Intimidate along with Memento are very key here for a successful sweep, making it near dead easy to set up Belly Drum and maximize its attack levels. The rest from there is basic, however, I put on Superpower, because I was lacking Fighting coverage, and Mawile already had Knock Off anyhow.
For the last slot, this was where I was really debating whether to actually pop on Klefki or not. If I did, the only thing I'd really get out of it is an additional hazard to stack, which don't get me wrong is nice, but this team already has plenty of hard hitters. I'd also be increasing my Fire weakness, which the team already has to be over-reliant on Diancie and Azumarill as it was. I also didn't exactly see a need for screens, since Whimsicott already had Memento in its artillery. So, with that said, I thought a scarfer would be extremely beneficial to have, as well as something to be able to hit Poison, and Gardevoir was added. Along with what I just mention, it also gives me even more support, in Trick to completely screw up opposing walls, and Healing Wish to recover any hurt teammates (which is great considering the only pokemon on the team with recovery at all is Togekiss). The only additional thing I think would be worth mentioning would be, the reason I'm running Psychic over Psyshock, because 1, slightly stronger, and 2, for Mega Venusaur--most are ran Physically Defensive, so Psychic would hit it a lot harder.