This boss will walk around the arena, periodically throwing one of three types of fireballs: green, red, or blue. He gets faster after every hit and dies after (by default) 5 hits.
The green fireballs turn into a Galoomba when they hit a wall. The Galoomba can be used to hurt the boss. The red fireballs disappear when they hit a wall. Both of the two hurt Mario and can be configured to turn into any sprite, normal or custom. The blue fireballs disappear when they hit a wall and freeze Mario for a short time if he touches them.
Make sure to specify the sprite numbers of the fireballs in larry.asm! Check the ASM files for customization options. If you give Larry more than 5 HP, make sure to expand the horizontal speed table.