Female Slime tank in challenge mode:
Press Y, R, R, U, L, L, Y, Down, Down, Down, Y, Select inside the church.

Helmet Slime tank in challenge mode:
Press Y, L, L, Y, R, R, Y, Up, Down, Select inside the church.

Unlock Knightro Tank for multiplayer:
Press Y, L, L, Y, R, R, Y, Up, Down, Select inside the church.

Unlock The Nemesis for multiplayer:
Press Y, R, R, up, L, L, Y, Down, Down, Down, Y, Select inside the church.