Note: Characters are unlocked in multi-player mode as you complete levels in story mode.
Complete the the indicated level in story mode to unlock the corresponding multi-player character.
Complete the indicated level to unlock the corresponding arena.
All you have to do is just play through Story Mode and eventually unlock characters.
Just play through Story Mode to unlock Multiplayer Mode Stages.
Each Larry the Titan you find in a level, unlocks concept art in the gallery extras menu.
Defeating Terrigon is similar to defeating Plasmus. The only thing that is different is that he has an electronic barrier. To break the barrier, stand on a green platform that will temporally short-out the shield. It will not take long; be ready and already hold the barrel you will throw at him through the barrier. You probably will not have time to hit him with another barrel before the barrier returns. Repeat the process until the team is ready for the team attack. Unlike all the other times, the team's placement is not near one another. They will form a circle around Terrigon.
This is a long fight. Slade fights you for awhile, then flees and lets his henchmen attack you. You must defeat all henchmen before he will come out of his barrier. Slade has demonic powers received from Trigon. It is better if you to keep your distance from him. When he uses his charged attack, it will cause serious damage to anyone up close, hitting them three or four times. He will attack you three or four times before you can finish him off with your team attack.
Cinderblock starts straight forward. Attack him until he has about half of his hit points left. An intermission sequence will start, where you will get sent into a hole in the ground. Defeat all the enemies in the new area and Cinderblock will attack you again. Then, do the team charged attack to finish him off.