Setting down a spawn point in the boss and using the /respawn command also works, and it summons all the users in a desired spot for the creator.
Additionally, a boss does not have to be a rectangular shape like in the picture. Some bosses require a much larger area, and the shape can often be found irregular. There may even be gravity arrows in the boss already put down, which at the right moment, will help guide players to the "safe" blocks.
Regardless of the kind of boss and who is "bossing", more often than not there will be "safe" blocks, which can be any color (Usually designated somewhere Ex. The minimap), for the players to land on when they first enter the boss.
Players will have to stay on the blocks of the color that won't be deleted and wait for the world creator/editor to change the blocks. Failure to do so will result in the losing players falling into the gravity arrows (or hazards, or portals, or booster arrows) below, and getting sent back to the key doors. Sometimes, there are hazards/portals/whatever somewhere inside the boss area that is not on the bottom, forcing players to avoid them while they are trying to stay on or move to the safe blocks.
The creator/editor will make some challenges for the players for they try stay on the not to be deleted blocks, because the creator/editor will delete the to be deleted blocks. Such challenges include the common Puzzle Types, or like above, forcing players to use the gravity arrows around them to get to the safe blocks.
The last player to stay on the boss without falling wins and has chance to get a coin, crown, or if the owner is run by a bot, one more number added to the player's own "win" counter. Each player must win the boss repeatedly to get more coins or wins. When a player has collected all the coins or has met some other criteria, that person may win the level. The reward could include a trophy, a crown, blue coins, edit rights or code, "admin" rights (If there is a bot), or even a tour around various types of art.
Often at times, many bosses offer the code or they involve winning many times. These bosses usually take a very long time to win due to the amount of coins that are required. As such, many players will get bored and leave before they rack up coins. Usually, only 2-5 coins are actually needed for a trophy (and the sense of accoplishment), but many players tend to add 7, 9, or even 12 coins to win, which is very tedious.
Additionally, repeating the same challenges over and over again will get boring very quickly and will make players leave. Players waiting at the boss while the boss is in session will also get bored and leave if there is nothing else to do.
Normally, people call green blocks "safe" and red blocks "unsafe", the latter being erased after a short time has passed. Please note that there are no official rules on a boss, and the "safe" and "unsafe" blocks can be any block the creater wants them to be.
There is also a certain bot spreading around that includes a "snake" bot. This type of bot involves only players placing down the green blocks, and the bot will automatically change the block to red, then erasing it. How quickly this could happen would depend on what the bot user would set it to. Note that snake bot users will often put down blue blocks (Or another colored block) which the bot will not detect, and therefore not erase, making blue blocks the "safe" blocks.