Now I don't say this often, but this archetype will dominate the meta alongside Metalfoes; it is already destroying the OCG. I can't deny that there is that sense of happiness seeing Kaiba's union monsters; some of you may remember my failed attempts at trying to make them work competitively, well now it seems impossible NOT to make it competitive.

We had our old XYZ series, followed by the even worse VW. Well now we have our ABC union monsters and welp it is one mean machine. To stop myself from rambling TOO much, I'll just point out the main cards in the deck.

Main Cards

A-Assault Core (LIGHT/Machine/Level 4/Union)
Once per turn, you can either: Target 1 LIGHT Machine-Type monster you control; equip this card to that target, OR: Unequip this card and Special Summon it. A monster equipped with this card is unaffected by your opponent's monster effects (except its own), also if the equipped monster would be destroyed by battle or card effect, destroy this card instead. If this card is sent from the field to the Graveyard: You can add 1 other Union monster from your Graveyard to your hand.

The usual equip to another union monster or un-equip to special summon the monster. While the old Y-Dragon Head had effects to boost the attack/defense, now A-Assault Core is unaffected by your opponent's monster effects. Furthermore, when it is sent from the field to the graveyard, you can recycle a union monster from the grave to your hand; fantastic. Run 3

B-Buster Drake (LIGHT/Machine/Level 4/Union)
Once per turn, you can either: Target 1 LIGHT Machine-Type monster you control; equip this card to that target, OR: Unequip this card and Special Summon it. A monster equipped with this card is unaffected by your opponent's Spell effects, also if the equipped monster would be destroyed by battle or card effect, destroy this card instead. If this card is sent from the field to the Graveyard: You can add 1 Union monster from your Deck to your hand.

No doubt the best out of the three, unaffected by spell cards is a great ability but the best part of B-Buster Drake is that when it is sent to the graveyard, you can search a union monster from the deck. Run 3

C-Crush Wyvern (LIGHT/Machine/Level 4/Union)
Once per turn, you can either: Target 1 LIGHT Machine-Type monster you control; equip this card to that target, OR: Unequip this card and Special Summon it. A monster equipped with this card is unaffected by your opponent's Trap effects, also if the equipped monster would be destroyed by battle or card effect, destroy this card instead. If this card is sent from the field to the Graveyard: You can Special Summon 1 Union monster from your hand.

Unaffected by traps + special summon 1 union monster from hand is not bad, but that sense of disappointment you are getting is because Buster Drake was just that good. Run 3

Gold Gadget/Silver Gadget (LIGHT/Machine/Level 4)
When this card is Normal or Special Summoned: You can Special Summon 1 Level 4 Machine-Type monster from your hand. If this card is destroyed by battle or card effect: You can Special Summon 1 Level 4 "Gadget" monster from your Deck, except "Gold/Silver Gadget". You can only use 1 "Gold/Silver Gadget" effect per turn, and only once that turn.

Similar effects, not only do you get to special summon a union monster when you normal summon one of these new gadgets; you can use it when you special summon it via their second Fire/Ice Hand-like replacement effects. Personally, I would run from 2 copies of each to 3 copies of each.

Union Hangar (Field Spell)
When this card is activated: You can add 1 LIGHT Machine-Type Union monster from your Deck to your hand. Once per turn, if a LIGHT Machine-Type Union monster(s) is Normal or Special Summoned to your field (except during the Damage Step): You can target 1 of those monsters; equip from your Deck to that monster 1 appropriate LIGHT Machine-Type Union monster with a different name from that monster, but that Union monster cannot be Special Summoned this turn. You can only activate 1 "Union Hangar" per turn.

A free search for any of the ABC union cards when activated, and a free attach when a union monster is normal/special summoned; I really don't need to explain how this is a good card do I? Run 3

Ties of the Brethren (Spell)
Pay 2000 LP, then target 1 Level 4 or lower monster you control; for the rest of this turn after this card resolves, you cannot Special Summon monsters, also Special Summon 2 monsters from your Deck with the same Type, Attribute, and Level as that monster, but with different names from each other and that monster. You cannot conduct your Battle Phase the turn you activate this card.

A decent card for the deck, you can essentially get out any of the main monsters (the gadgets, ADC monsters). Of course while you cannot special summon monsters the turn you resolve this, you can still use it with Union Hanger to help set up. Personally, I think it's one of those run 3 for consistency or 0 cards; but I'll have to do more testing on that. For now I'll say run 0.

ABC-Dragon Buster (LIGHT/Machine/Level 8/Fusion)
"A-Assault Core" + "B-Buster Drake" + "C-Crush Wyvern"
Must first be Special Summoned (from your Extra Deck) by banishing the above cards you control or in your Graveyard. (You do not use "Polymerization".) Once per turn, during either player's turn: You can discard 1 card, then target 1 card on the field; banish it. During your opponent's turn: You can Tribute this card, then target 3 of your banished LIGHT Machine-Type Union monsters with different names; Special Summon them (this is a Quick Effect).

Not only is it very easily to get out, you can banish any card by discarding one card from the hand. You can also tribute to special summon the ABC union monsters during your opponent's turn. Since it is a Quick Effect, you could chain it; once ABC-Dragon Buster is out, it is going to be difficult to destroy. Run 3

Deck Skeleton

3 x A-Assult Core
3 x B-Buster Drake
3 x C-Crush Wyvern
3 x Photon Thrasher
2 x Gold Gadget
2 x Silver Gadget
3 x Terraforming
3 x Union Hanger
3 x Pot of Cupidity (good luck getting it IRL)
1 x Reinforcement of the Army
3 x Dimension Barrier
3 x Anti-Spell Fragrance (if you hate pendulums)

Weakness

Stopping Union Hanger will hinder the speed of the deck greatly.
Stop cards from being banished: Chaos Hunter etc
Light-Imprisoning Mirror
Generic 'stop special summon' weakness
All the monsters are machines, why hello Cyber Dragon
Weak to field clears (we have 1 Raigeki and 2 Dark Hole in the TCG)

Side cards

Vanity's Emptiness
Cyber Dragon
Ghost Ogre
Maxx C
Light-Imprisoning Mirror
Chaos Hunter
Any Kaiju monster (an out to literally everything)
Imperial Iron Wall
System Down

That is everything for now, hope you enjoyed the guide and have fun with them. If you have any questions just send me a PM or post it here.