Are there any other Guardians?

Solution 1:

The bosses you are looking for are:

Valukar

This boss is the Guardian of Fire, whom protects the Summon Daedelus. This boss can be found in the Yampi Desert, near Alhafra.

Dullahan

This is the Guardian of Earth, and protects the Summon Iris. This boss is found in the Anemos Sanctum, near Contigo.

Additionally, there is a grand total of 13 multi-elemental Summons throughout the game that can be found (not including the 4 guarded Summons):

• Zagan (1 Venus, 1 Mars)
• Megaera (1 Mars, 1 Jupiter)
• Flora (1 Venus, 2 Jupiter)
• Moloch (2 Mercury, 1 Jupiter)
• Ulysses (2 Mars, 2 Mercury)
• Haures (3 Venus, 2 Mars)
• Eclipse (3 Jupiter, 2 Mercury)
• Coatlicue (3 Jupiter, 3 Mercury)
• Charon (8 Venus, 2 Jupiter)

Interestingly, Charon is actually the Ultimate Venus Summon, not Iris (Ultimate Mars Summon), which is the Summon guarded by the boss in Anemos Sanctum, but can be found near the entrance of the Anemos Sanctum. The only pre-requisite is that you have all 72 Djinni to access the Sanctum, including all 28 from the original Golden Sun. This does seem a bit odd, since Daedelus is not actually an "Ultimate Summon".

Solution 2:

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.