This is a non FWG defensive core that allows you to sponge hits and spread status until your offensive pokes' counters and checks are weakened or crippled.
The core has strong type synergy, together resisting hits of all types except Fire and Water, and covering each others' weaknesses well.
The core is resistant/immune to entry hazards.
The core attacks with Psychic, Poison, Fire, Electric and Ice moves which provide good coverage in combination.
Sigilyph @ Flame Orb
Magic Guard
Timid
252 HP / 172 Spe / 86 SpD
-Cosmic Power
-Stored Power
-Psycho Shift
-Roost
Anything this weird and ugly has to be damn useful to find its way onto teams. Immune to status after one turn, as well as capable of burning things left right and centre. Burned enemies with SE attacks are no longer scary (just be careful about Psycho Shifting burns when Heracross is around), and Sigilyph can Roost away 3 of its weaknesses with good speed. Once it has a chance, Sigilyph can start setting up Cosmic Powers that boost defences and the Stored Power that it has.
172 Speed EVs allow it to outspeed neutral natured base 100s like Zapdos and positive natured Rotom formes, allowing Sigilyph to Roost to remove its Electric weakness before they can hit it.
Defensive Synergy:
4x resistant to Fighting for Magneton
Immune to Ground for Magneton
Resistant to Psychic moves for Weezing
Burns Physical attackers
Absorbs status, can throw Para/Burn/Poison back at enemy
Note: if you want to use Xatu instead, it has the same typing but isn't as sturdy, and both pokemon can reflect/are immune to status. Therefore it is only recommended if you're very keen to reflect entry hazards.
Weezing @ Black Sludge
Levitate
Bold
252 HP / 96 Def / 160 SpD
Clear Smog
Flamethrower
Pain Split
Toxic/Will-O-Wisp
Making up the second part of the core is everyone's favourite website mascot evolution, Weezing.
Weezing supports the team with physical bulk, stat elimination through use of Clear Smog (which is stopped by Steels but has ok coverage with Flamethrower) and by spreading status. Toxic is preferred as Sigilyph has a more accurate Burn move. Also, Weezing is already resistant to many physical sweepers, and Flamethrower is SE against the many Grass/Poison types in UU, as well as Toxicroak, so poison immune pokes better watch out! Pain Split for stalling while Toxic does its work.
Note: You could also use Stockpile but it doesn't go well with Clear Smog, so then Venoshock becomes an option.
Defensive Synergy
2x resistant to Fighting for Magneton
Immune to Ground for Magneton
Only weakness Psychic is covered by Sigilyph and Magneton.
Takes Fighting, Bug, Poison and Grass hits like 'a champ'.
Magneton
Eviolite
Magnet Pull
Bold +Def/-Att
252HP / 252Def / 4SpD
Substitute/Thunder Wave
Metal Sound
HP Ice/Thunder Wave
Discharge/Thunderbolt
This is what happens when you leave magnets too close to one another, they get stuck together. If we were to leave them alone any longer, they might even turn into an alien!
Magneton is best known for trapping Steels, however they're not that common in UU. You'll most often find Steelix, who should not be dealt with by Magneton, or Registeel, who is a piece of cake after a Metal Sound or two. Substitute for protecting against status and incoming checks/counters.
Discharge for spreading paralysis, and it packs a punch with Metal Sound. If you need reliable paralysis use Thunder Wave. Metal Sound is great for forcing switches and turning special walls into paper.
Note: You could also go on the offence with 252Spa/252 Spe, Leftovers/Life Orb and Thunderbolt, but since this is a defensive core I am focusing on MagneTON.
Defensive Synergy
4x resists Electric for Sigilyph
2x resists Dark, Ice, Rock for Sigilyph
2x resists Psychic for Weezing
Its partners are immune to its 4x weakness, Ground
Its partners are resistant to its weakness to Fighting
Still has a Fire weakness that needs covering
Comments and criticisms are most welcome!