Party was a mechanic introduced in Zendikar Rising. Cards with a party effect would have reduced costs or increased potency if you had some cleric, wizard, warrior, and/or rogue in play.

The mechanic sucked.

However, to make the most of it, I flooded this deck with Changeling creatures; they have every creature type including the party-core types of cleric, wizard, warrior, and rogue. Although a single changeling can only count towards one member of the party, having 14 changelings in the deck dramatically increases the consistency of completing the party as early as turn 3.

Strategy

Get an opening hand with at least one land and one mana dork. Then, start playing stuff. Optimally, you want to form a party and immediately use Nimble Trapfinder to draw into more stuff to play. The more stuff, the cheaper your big creatures (Tazri, Beacon of Unity; Veteran Adventurer; Zagras, Thief of Heartbeats; Valiant Changeling) become.

Card Justification

Universal Automaton is a generic 1/1 changeling. Though unsuspecting, it is fundamental to completion of the party.

Changeling Outcast is a black 1/1 changeling. Though it cannot block, its evasion allows you to chip in damage to whittle your opponent and provide easy triggering of Nimble Trapfinder.

Mothdust Changeling is a blue 1/1 changeling. It can fly, which is also useful for evasion.

Valiant Changeling can cost as little as {W}{W}, and a 3/3 doublestriker is pretty good.

Of course, other party-core creatures without changeling have to be included. These creatures have evasion, mana abilities, and/or party effects.

The big creatures are discounted by your perty; Tazri, Beacon of Unity; Veteran Adventurer; and Zagras, Thief of Heartbeats can be played fairly early (turns 3-5) and can really wear down your opponent.

Lastly, Strength of Solidarity is the only noncreature spell. +4/+4 for one green is very appealing when placed on an unblockable or flying creature.

Budgeting

The deck is already optimal in budgeting, but if you want to spend more, include these cards:

Ancient Ziggurat and City of Brass provide easy color fixing.
Weathered Wayfarer is a cleric AND draws your lands for cheap and efficiently.