The Lobster King introduces itself by charging onto the bridge. It will then wait north of the bridge, behind one of the switches, throwing ranges projectiles at a long distance. The enemy generally remains outside of melee range, being vulnerable only when it attacks or otherwise approaches the bridge.
When you approach the lobster and pull the switch, the boss will take 200 damage be frozen for a few seconds. As the lobster can remain out of melee range, it's best to use the switch as it approaches.
The lobster will also wrap itself against the bridge segments, and tear it apart, destroying the segment.