When you enter a Scenario simply right click your portrait up the top and select Leave Scenario Party. You will gain no exp or renown from players that you haven't tagged but the ones that you kill give you MUCH more exp and renown than if you were in a group. Basically leave the scenario party and tag lots of players with fast abilities and watch yourself hitting the top of the EXP/Renown gained charts in every Scenario, no matter if you win or lose.