Splody

Use the level editor to set a map that has a center area surrounded by indestructible bricks, and place your spawn point inside this area. Here is an example on how to create your level:
 

Set up an offline game in “Free for all”, the Player + Bot Count to 31, and Bot Difficulty to “Hard”. While you’re in the stage select, select your custom stage and in Advanced Options, have Postmortem set to "Nothing", Round Time to 0:00, and Player Start to “Fixed”. Do nothing and let the bots kill themselves or the stage close on them.