To unlock The Nemesis tank in Challenge mode, go inside the church and press Y, R(2), Up, L(2), L, Y, Down(3), Y, Select.
Giggles in Tank Masters mode: To unlock Giggles in Tank Masters mode, beat the game, then talk to "Meggan", the green Egg Slime in the northwest building of the central part of town.
Gott Schliemen in Tank Masters mode: To unlock Gott Schliemen in Tank Masters mode, defeat the final Boss and rescue all 100 Slimes. Then, defeat Hooly and the Shogun in the last round of Tank Masters mode.
Schwarzen in Tank Masters mode: To unlock Schwarzen in Tank Masters mode, defeat the final Boss. Then, go to where you defeated Slival on Mt. Krakatroda. Defeat Slival again.