Here are some ways 2 get new mosters:
Godzilla2000: beat adventure mode w/ Godzilla90`s(easy lv.)
Destroyah: beat adventure mode w/ Godzila2000.(easy lv.)
King Gidorah: beat adventure mode w/ Mechalon(easy lv.)
Mecha King Gidorah: beat adventure mode w/ King Gidorah(easy lv.)
Gigan: beat adventure mode w/ porqupine dude(easy lv.)
Rodan: beat adventure mode w/ Gigan(easy lv.)
Mecha Godzilla: beat adventure mode w/ all monsters(easy lv.)
Organ dude: beat adventure mode w/ all mon.(easy lv.) then beat adventure mode w/ Mecha Godzilla(medium lv.)