Note: This game is also titled Slime MoriMori: Dragon Quest 2 - Daisensha To Shippo Dan.
Go inside the church and press Y, L(2), Y, R(2), Y, Up, Down, Select to unlock the Knightro tank in Challenge mode.
Go inside the church and press Y, R(2), Up, L(2), L, Y, Down(3), Y, Select to unlock The Nemesis tank in Challenge mode.
Successfully complete the game. Then, talk to "Meggan", the green Egg Slime in the northwest building of the central part of town.
Defeat the final Boss and rescue all 100 Slimes. Then, defeat Hooly and the Shogun in the last round of Tank Masters mode to unlock Gott Schliemen in Tank Masters mode.
Defeat the final Boss. Then, go to where you defeated Slival on Mt. Krakatroda. Defeat Slival again to unlock Schwarzen in Tank Masters mode.