High Summoner Galatea:
I have several ideas regarding potential opponents for you.
First, you should try and find Summoner Almors. He has been sick lately and is recovering on Talking Island.
You should also try Summoner Basillia. I have heard that she is busy researching a new form of Summoning magic close to a farm on the southern shores of Gludio.
Summoner Camoniell, of the Elves, has been in seclusion in a cottage in southern Dion. I've heard he has had some trouble lately.
Summoner Celestiel usually resides in the Ivory Tower, but has returned to the Elven Woods on urgent business from her homeland.
The Dark Elves, Summoner Brynthea and Summoner Belthus, are savvy opponents for you, but they are hard to find, as they do not stay in one place very long. Last I heard, Brynthea was somewhere in the northeastern parts of Giran, and Belthus was somewhere in the Cruma Marshlands...
These Summoners are proud and haughty people, and may scoff at a challenge from an apprentice Summoner like yourself. But, at the same time, they will likely accept your challenge, so that they may display their superiority over you in front of their friends, family, relatives, on-lookers and animals.