I think that we need to have some structure, so that a shadow wizard doesn't overpower the duel. Furthermore, this should be done without ever needing to ask the advanced player to slow down. Relying on politeness is not the solution.
I see a couple of options:
This only applies to automatic team-up. Obviously, if you select your own people to go with you, there are other factors besides level going on.
Set the range of allowed levels by world. These rules would be inflexible. For example, Wizard City Dungeons could be capped at level 20, Krok at level 30, with level caps set for each world.
Let the person who presses the Team-Up button choose the maximum level allowed, in multiples of 10. This number would then appear on the team kiosk for that request. When using this option, a warning could appear reminding people it might take longer to assemble a team.
