The Penguin UFO Boss appears when you put a distress beacon on the ground and interact with it.
You will be asked to make the distress beacon after a few quests at the beginning of the game. It is meant to help you out but instead attracts some unwanted attention in the form of a boss fight.
The UFO itself isn't the only thing that you have to care about. It will continuously spawn penguins on the ground. At the beginning they are easy to fight off. Later on theybecome stronger, throwing bombs and other projectiles. Try to not stand under the UFO. Occasionally, it will try to smash everything beneath it by shooting straight down. If you do not engage The UFO for some time it will leave. Make sure to equip armour since it hits around 80 with full copper armour.
The UFO has a pattern of movement: it drops off penguins, battles against the player at a close range, then leaves, comes back, and repeats. You can disrupt this pattern by boxing it in with blocks. When it tries to retreat, it will get stuck and not attack, at which point you are free to kill it at your leisure (Note, the UFO is now able to break blocks, wich may prevent this strategy from working effectively).
It is wise to establish a battleground at the spawn point, so you can return to battle quickly after dying. A good feature to include in your base is a series of one block openings in a ceiling, allowing you protection while you shoot directly upwards, in the fashion of the age old Space Invaders technique.
The UFO is incredibly weak to bombs, taking about 500 damage per bomb. You can easily kill him with 7-8 bombs total.
- The boss is confirmed to be on Pre-Alpha version of the game and can be seen here at 14:30, just a few seconds after the placement of the Distress Beacon.