Battle Bills are Bullet Bills that do more than just be launched and hope to hit you. These guys fight you and hope to hit you. They look like Bullet Bills with red eyes (green on the red variation)
They are great fighters indeed. They will follow you if they see you. However, they cannot see you if you go through a hidden tunnel, which will make them be confused for a while and then go away. If you can make them hit a wall, then they will be stunned a bit. It takes 3 wall hits or jumps on the head to destroy these. If one runs into a block, they will destroy it, but take a hit in the process. If they hit a ? block, then they will make the item fall below and the ? block will be destroyed and the Battle Bill will take a hit. They are rarely found shot out of Battle Bill Blasters, as they normally fly in the air.
Normal ones will just follow you. However, there are others.
Red ones will use flamethrowers.
Light blue ones breath ice.
Yellow ones make branching lightning bolts.
Dark blue ones toss boomerangs.
Green ones whip with spiky vines.
Purple ones spit blobs of poison that splatter.
White ones toss bombs.
Battle Bill Blasters
These are rare, as stated above. The only levels they are in (by the way Battle Bills are only in More New Super Mario Bros) are level 7-3, level 8-6, level 8-10 (mid boss castle), world 20/all levels Blasted Bills, getting hit by one on the map of world 6, and have a cameo in the start of the game. Belive it or not, it is possible to destroy these. You must jump on it 10 times to do so, though. Some Battle Bill Blasters will target you, some willl be like regular Bullet Bill Blasters, and some will have wings or wheels and chase you, or they may have wings and be able to fly. Some are operated by Monty Moles, though. If you take out Monty, you can actually control the Blaster! This is a tactic in the minigame Blasted Bills.