If you're looking for anime similar to Guardian of Darkness, you might like these titles. All recommendations are made by Anime-Planet users like you!
While investigating the site of a strange explosion with his friend Tetsuro, Sho Fukamachi stumbled upon a mysterious artefact – the bio-armor suit called Guyver! Unfortunately, the owners of the artefact – a shadowy organization called Chronos Corporation – want it back; and they want to ensure that no one lives to tell of its existence. To survive, Sho must unlock the increasingly fearsome powers of the Guyver suit while battling an endless stream of Chronos' bio-engineered monsters. His journey will lead him to the truth behind not only the origins of the artefact, but also of mankind itself!
The only anime that I can think of that will come close to this one is Guyver- The Bioboosted Armor. In both anime the protagonist has acces to an armor that grants immense power to the user, and enables them to combat monsters and the likes.