At the exact moment that
night starts, run into the dungeon and wait for a head to summon. Then, lure
it outside to your battlefield. The heads have 6000HP, just like Skeletron's
head, aqnd when defeated give the same effect of beating the full Skeletron.
This means that if you can beat the head (which is way easier than the full
body), the dungeon will open up, and the game will act as if you've beaten
Skeletron normally.