The best way to utilize Oni is in one on one combat since his strength levels and weapons allow him to defeat some enemies almost immediately. In addition to that, he is also extremely fast which is useful during the gameplay. Physically, Oni is very strong and equally as tall.
He can be bought in the app store for 4.99 dollars or unlocked after finishing the campaign in the Steam version of the game. Regarding the original game, the best hero would probably be Oni. Kingdom Rush: Vengeance best heroes Kingdom Rush best heroes