To beat Tricephalos in Elden Ring Nightreign (also known as Gladius, Beast of Night), you need to exploit its main weaknesses and pick the best possible classes going in. Here's a quick precis of the boss fight, before we go into more detail momentarily:

The Tricephalos boss named Gladius, Beast of the Night, is the first that you'll face in Elden Ring Nightreign, so let's figure out how to beat it.

The Tricephalos boss is the real proving grounds of Elden Ring Nightreign, and many of the game's features are hidden behind beating it.