Here is a hack which increases the difficulty of the game by throwing harder types of enemies at you during the gameplay.
Here is a demo video on the hack
Teenage Mutant Ninja Turtles II.nes - NOINTRO
CRC32: C9FFBBDB
MD5: 63D85DB88F52E9A1C3DA2E7813E5B0E1
SHA-1: 40FF068D2A7BE1202D843C2A9DC1220F087A094A
SHA-256: 637FA9D773CB39FCD4E55FAF46BC53F82AC9D7706F8473ADEB06E2B9E95C2414