There are 7 places to go to:

Nikolais world:

In the Forest Village where Nikolai is from. After finishing his side-quest, return there and speak to the man at the entrance building up top.





Bens world:

1) Siope monastry - there is one silent dude roughly in the middle top of the area to speak to.



2) Akos: Right when you enter, turn upwards, where the children are playing. There is a house with an open door, slightly hard to spot, inside is a woman to play with





Cordelias world:

1) In Feorlan: You can play against the deputy (Theo) that Cordelia hired earlier in the story.



2) Leornia High Street: There is a fancy looking guy in front of a building.



Alexandras world:

Barbarian Camp: A woman on the right side of the area plays with you.



Your Airship:
After finishing Ziordias quest, you can find the Barbarian shop on your ship. Dreyfuss will play with you.






In the first version, when you returned to the Explorers Camp after playing against all these people, you get rewarded with
A Mega-Elixir

Apparently, this changed. The Developer said: " In v1.2 update you get one extra goodie for completing it. Should hopefully be nice for players who bother to complete it."