After you make it past Hinopio's shop in Barrel Volcano, you encounter the Czar Dragon.
Defeat Czar Dragon
The Czar Dragon is weak to ice and strong against fire. It can use Water Blast, hitting all character for high damage. It can use Flame Wall, which does moderate damage. It can use Iron Maiden, which inflicts Fear status on a character. It can summon four orbs that float next to it, and these orbs will randomly hit a character for low damage. You can attack the orbs to get rid of them.
After you defeat Czar Dragon, it is replaced by Zombone. You can deal damage with Pure Water if you have any. Czar Dragon is weak to Thunder and Jump attacks. It can cast Boulder, hitting all characters for high damage. It can cast Storm, dealing high damage to a single character. It can use Scream, which inflicts Fear status on a character.
After the battle, you can go northeast across the newly formed bridge to reach the next room. Go toward the Star Piece. After the cutscene, go up the room and through the door at the end. There will be another cutscene. Go up and through the door at the top. Keep following the rangers, and eventually you will see the rangers jump onto a spring. Jump on the spring and you will battle the Axem Rangers.