Maybe Top 4 OR Top 2. The league can be like this (only if he uses Unova pokemon):
Round 1: (3 on 3) Bold means the pokemon who won
Ash vs CotD : Gigalith, Servine, Samurott vs Unfezant, Scolipede, Sawsbuck
Trip vs CotD: Chandelure, Galvantula, Jellicent vs Swanna, Simisage, Darmanitan
Burgundy vs Kenyan: Samurott, Simisear, Stoutland vs Zebstrika, Sawk, Klinklang
Bianca vs CotD: Lilligant, Alomomola, Emboar vs Garbodor, Basculin, Eelektric
Luke vs CotD: Leavanny, Golett, Larvesta vs Archen, Sismitoad, Serperior
N VS Cheren: Sigilyph, Tirtouga, Liepard vs Swoobat, Crygonal, Druddigon
CotD vs CotD: Ferrothorn, Haxorus, Scrafty vs Watchog, Gothitelle, Karrablast
CotD vs Langley: Elgyem, Klang, Pignite vs Simipour, Bisharp, Beartic
Round 2:
Ash vs Bianca: Emboar, Cubchoo, Pikachu vs Cincinno, Musharna, Gothorita
Trip vs CotD: Serperior, Vanilluxe, Conkeldurr vs Whimsicott, Reuniclus, Amoongus
N vs Luke: Reshiram vs Zorua, Escavelier, Golett
Burgundy vs Langley: Sawsbuck, Haetmor, Bouffalant vs Beeheeyem, Audino, Bisharp
Round 3 (Full battle)
N vs Langley: Cofagrigus, Reshiram vs Beeheyem, Audino, Bisharp, Beartic, Crygonal, Simipour
Ash vs Trip: Unfezant, Scrafty, Servine, Wildcard, Seismitoad, Emboar vs Eelektross, Unfezant, Serperior, Jellicent, Chandelure, Conkeldurr
Finals:
Ash vs N:
Leavanny vs Tirtuoga (Leavanny)
Leavanny vs Sigilyph (Leavanny returns)
Gigalith vs Sigilyph (Gigalith)
Gigalith vs Liepard (Liepard)
Leavanny vs Liepard (Leavanny)
Leavanny vs Archeops (Archeops)
Seismitoad vs Archeops (Seismitoad and recalled)
Cubchoo vs Cofagrigus (Draw)
Seismitoad vs Reshiram (Reshiram)
Pikachu vs Reshiram (Reshiram)