I think It will be a tie between Mega Sceptile and Ash-Greninja. Notice how Ash calls Pikachu back and sends Noivern out to battle Salamence? Then Goodra vs Slurpuff? After Salamence and Slurpuff gets defeated, Shota will have 1 pokemon left which is Mega Sceptile and Greninja will battle it. Ash will have 2, which is Pikachu and Greninja.
So far this is how the matches went.
Slaking vs Hawlucha ----
Slaking vs Talonflame----
Talonflame vs Clawitzer----
Clawitzer vs Pikachu----
Pikachu vs Aegislash----
Pikachu(switched out)