Are there any other Guardians?

According to Golden Age's Wikia, there are four superbosses, optional, in the game.

Sentinel is one of four "superbosses" located throughout Weyard in The Lost Age, the others being Valukar, Star Magician, and Dullahan. All of these bosses fight using unique battling setups and are very tough.

(From Sentinel page)

The two other bosses you haven't defeated yet are :

• Valukar : probably the Guardian if Fire, as it is stated:

The name "Valukar" may be based on Valaraukar, which translates to "demons of might", and was an alternative name for the Balrog race in Tolkien's mythos. This is further supported by Valukar's appearance as a red, winged demon associated with fire.

It is located at the end of the optional dungeon Yampi Desert Cave.

• Dullahan : located at the end of the optional dungeon Anemos Inner Sanctum. The Wikia also states that it will give you the most powerful summon sequence, while being the hardest boss to defeat.

There's no etymology for that name, so I can't be sure it is the Guardian of Earth. There is no other super boss though.

All provided links to Wikia articles contains all details concerning their arsenal, skills and characteristics, but I thought it would have been irrelevant here.