Charizard-Y: I was running a set of Blue Flare/Aeroblast/Quiver Dance/Roost with the ability Magic Guard and a Toxic Orb. Rock types are effectively absent as far as I can see and, beyond those, only Zekrom, Aerodactyl, and Megampharos are the only things worth noting that resist his STABs (Flash Fire aside). As such, Charizard-Y can simply add Ice Beam to his arsenal and have neutral coverage on all relevant non-Flashfire threats. Sure, Lanturn will still resist all three, but... Lanturn probably won't resist much for long anyway.
Charizard-Y sits at base 100 speed, hence I prefer Quiver for the speed boosts since there's a few, significant threats that are faster, namely Mega Aerodactyl. Of course, Tail Glow and a Timid nature are also viable if you'd rather have more power. If Charizard-Y wants coverage, the best option without sacrificing much power is to probably swap Aeroblast for Oblivion Wing and drop Roost. You lose the ability to heal in the face of certain coverage moves, but boosted Oblivion Wing is quite handy for HP recovery.
Life Orb is another option for more power, as is expected on a Magic Guard set. But being safe from the still prevalent sleep is not easy to give up.
And because of his typing, Charizard-Y can take on common threats like Pixel Xern, Refrige-Kyu-B, and Mewtwo X. It's worth pointing out that Charizard's Aeroblast at +1 will deal 80% damage to a +2 Sp D Xtwo, just to give you an idea of how hard he hits.
Absol: The hard part about Absol is that it's a bit on the frail side and it's not particularly fast until it evolves. It can take neutral, unboosted hits from most attackers as well as resisted hits, but it better be able to megavolve and dish out a ton of damage back in order to make staying in against such threats worthwhile.
As said above, I was running Simple in base form. I tried both Shell Smash and Shift Gear and found the latter to be somewhat superior while under Simple.
Absol suffers from four-move slot syndrome pretty badly though. It wants set-up, Crunch, Pursuit, Sucker Punch, coverage, and recovery. Crunch for general use, Pursuit since it can force a lot of switches (especially if it sets up in something like Giratina's face), Sucker Punch for smacking those Quiver Dancing and Contrary Psycho Boosting Y-twos, and coverage so it can hit Xern and Tyranitar (Iron Tail/Head works great here), ideally on the switch for the former. Wish-passing can alleviate this problem a bit and, fortunately, Absol and Cress cover each other pretty well since the only common Bug-move of note is U-Turn.
But once you're set-up and megavolved, Absol can tear things apart. Because of it's fraility and susceptibility to certain priority users, it makes for a better late-game sweeper once certain threats are weakened or removed. Especially since it can only Simple Boost > Megavolve only once, so you're going to want it to count.
Also, as said, many "lolboosters" are either Ghost or Psychic. Absol can bounce Topsy by megavolving in their face and KO them on the same turn, which often prevents them from following-up with Parting Shot. Haze and Heart Swap bypass this and Unaware can cause problems, so you'll want those users to be weakened or removed.
Skymin: Getting it in was tough with the way my team was set-up. But, once it was in and the threats were removed, Skymin's a beast. After a Quick Feet boost, it outspeeds the entire unboosted metagame, Deo-S included, with a Modest nature. Hence, it only needs to worry about priority attacks or hits from things it can't OHKO.
After a Tail Glow boost, Skymin's STAB Oblivion Wing hits much harder than I expected and recovers huge swaths of HP. Even after reducing my Sp. D EVs so I could OHKO Imposter switch-ins with Ice Beam, Skymin still had just enough bulk to take unboosted hits and then heal off the damage with Oblivion Wing. It doesn't matter much if it can be 2HKOed or 3HKOed a by a lot of things if it keeps popping it's HP back to full. Especially if it's simultaneously threatening a 2HKO back.
The set I settled on was Oblivion Wing/Ice Beam/Tail Glow/Leech Seed. Because Water, Rock, and Ground types are rather uncommon, Giga Drain saw almost no use. Should they make a return, it wouldn't be a bad idea. Leech Seed came to mind after I facepalmed when I mispredicted and Psycho Shifted a burn to an Imposter switch-in (luckily I won the ensuing speed tie). Most victims will be giving Skymin enough HP to overcome the burn damage, or more, and it also lets it stop the recover of PHers and KO any variant of Sheddy. Oh, and if Leech Seed is bounce, what does Shaymin care? It's a Grass-type, those seeds aren't working!
Steel-types are problematic for this set, so Blue Flare, Aura Sphere, Earth Power, or Secret Sword can all help there. Kyu-B, while it doesn't like switching into a boosted attack, provides a very obvious threat to Skymin. Luckily, because Kyu-B is more predictable than a guy who's telling you what he's about to do, it's actually rather unlikely to KO Skymin so long as there's a safe-switch in (Heatran, Aegislash, Skarm, Cress, Scizor, etc.) Honestly, Skymin went down to unexpected stuff more often than anything else, like a Stored Power Prankster Deo-D (wait wut?).
But, like Absol, this would probably make a better late-game sweeper after you've removed key threats and the remaining opposition is unable to muster enough offense to take down Skymin.
