Baumbas are hostile mob that spawn in a structure called the Space Arena. They fire explosive projectiles at the player, and these explosions have sufficient knockback to pose a serious threat of being knocked out of the arena. The ideal method of combating the Baumbas is via long range weapons, such as Sniper Rifles and Bows.
If you're trying to get SpaceKing Armor, it is recommended you go after the Reflucts under the Space Arena as explosions can destroy items.
Note: If you are using version 2.4.B or below, the SpaceKing Leggings will never drop, due to a bug. This is fixed in the latest version.