Parties in the Map Editor have a useless and misleading field
There's a "configuration" set of fields for parties in the map editor. However, there is also a configuration entry for each unit in the party, and it overrides the party configuration.
The party configuration leads one to believe it will allow you to set the entire party's level or buffs with one entry, but it doesn't work. You should either add a check box to make it override the individual configuration settings or remove it.