How is the experience rewarded? Is it only while fighting them or can I keep other low defense options in my choices and get the bonus xp on them as well?

Currently i'm using;

Ice Troll King
Moss Giant
Khazard warlord/Black knight Titan
King Roald
Count Draynor

Wondering if I can add Argg, as well as stuff like Kendal etc and still be getting max xp with a bit more points.