Composite Golem goes well with Nim Deathmantle if you cannot afford omniscience, as said combo grants infinite mana.
Not sure if this would work, but Spy Kit might be your answer. There are also the wish type spells and Fae of Wishes to search for cards from your collection. The question is the definition of “card”. Using the wish cards with As foretold means you would be able to cast cards with no mana cost, be it Living End or hopefully something like Garruk the Slayer. Obviously none of this last part would be tournament legal, but you might be able to argue that MLP cards are still cards and search them with Wishes.
Since four of the ponies don't exist yet, you can try to Conflux for Rarity, _____, Evil Twin, Dack's Duplicate, and Altered Ego instead, and use Composite Golem mana to change their names.
With Omniscience out, you can alternatively try to win by drawing through your deck with enchantress triggers and swinging with the angels from Sigil of the Empty Throne + Temur Ascendancy.
1 Princess Twilight Sparkle
Creature - Friendship Is Magic (6):
1 Applejack
1 Fluttershy
1 Nightmare Moon
1 Pinkie Pie
1 Rainbow Dash
1 Rarity
Creature (10):
1 Academy Rector
1 Argothian Enchantress
1 Composite Golem
1 Eternal Witness
1 Mesa Enchantress
1 Nessian Wanderer
1 Satyr Enchanter
1 Setessan Champion
1 Tuvasa the Sunlit
1 Verduran Enchantress
Sorcery (4):
1 Conflux
1 Diabolic Intent
1 Flood of Tears
1 Praetor's Counsel
Instant (4):
1 Eladamri's Call
1 Enlightened Tutor
1 Muddle the Mixture
1 Mystical Tutor
Enchantment (38):
1 Abundance
1 Abundant Growth
1 Cast Out
1 Control Magic
1 Darksteel Mutation
1 Dawn's Reflection
1 Destiny Spinner
1 Doomwake Giant
1 Dryad of the Ilysian Grove
1 Dueling Grounds
1 Eidolon of Blossoms
1 Enchantress's Presence
1 Energy Flux
1 Exploration
1 Fertile Ground
1 Imprisoned in the Moon
1 Land Tax
1 Mystic Remora
1 Oblivion Ring
1 Omniscience
1 Overgrowth
1 Quarantine Field
1 Rhystic Study
1 Sandwurm Convergence
1 Seal of Cleansing
1 Seal of Doom
1 Seal of Primordium
1 Sigil of the Empty Throne
1 Smothering Tithe
1 Soul Snare
1 Sphere of Safety
1 Stasis Snare
1 Stony Silence
1 Stranglehold
1 Sylvan Library
1 Temur Ascendancy
1 Trace of Abundance
1 Utopia Sprawl
1 Wild Growth
1 Blood Crypt
1 Bloodstained Mire
1 Breeding Pool
1 Command Tower
1 Exotic Orchard
1 Fetid Heath
1 Flooded Grove
1 Flooded Strand
5 Forest
1 Godless Shrine
1 Hallowed Fountain
2 Island
1 Krosan Verge
1 Mana Confluence
1 Mountain
1 Murmuring Bosk
1 Overgrown Tomb
3 Plains
1 Polluted Delta
1 Reflecting Pool
1 Sacred Foundry
1 Serra's Sanctum
1 Steam Vents
1 Stomping Ground
1 Swamp
1 Temple Garden
1 Twilight Mire
1 Watery Grave
1 Windswept Heath
1 Wooded Foothills
Princess Luna Pile (6):
1 Castigate
1 Disciple of Deceit
1 Future Sight
1 Hallowed Moonlight
1 Oath of Liliana
1 Ponder
Horde of Notions | Azami, Lady of Scrolls | Mirri the Cursed | Kaalia of the Vast | Ghave, Guru of Spores | Nin, the Pain Artist | Damia, Sage of Stone | Derevi, Empyrial Tactician | Vela the Night-Clad | Jhoira, Weatherlight Captain | Nahiri, the Lithomancer | Princess Twilight Sparkle
Not sure if this would work, but Spy Kit might be your answer. There are also the wish type spells and Fae of Wishes to search for cards from your collection. The question is the definition of “card”. Using the wish cards with As foretold means you would be able to cast cards with no mana cost, be it Living End or hopefully something like Garruk the Slayer. Obviously none of this last part would be tournament legal, but you might be able to argue that MLP cards are still cards and search them with Wishes.
Horde of Notions | Azami, Lady of Scrolls | Mirri the Cursed | Kaalia of the Vast | Ghave, Guru of Spores | Nin, the Pain Artist | Damia, Sage of Stone | Derevi, Empyrial Tactician | Vela the Night-Clad | Jhoira, Weatherlight Captain | Nahiri, the Lithomancer | Princess Twilight Sparkle