difficult to say if you have done something wrong or not, but it seems your strategy could work well
It is not a difficult scenario (we have done at level 5 (+1 with respect to our standard difficult) without problems.
Actually we are noticing that after some scenarios if you do not increase the difficult the game becomes too easy
Mainly because you start knowing the gameplay strategy and also the cards of your friends
I agree on the game eventually getting easier as you get more comfortable with it. What my group has started doing is when we win a scenario we bump the difficulty up 1 for the next scenario, and if we lose we bump it back down. We've been sitting at 7 for a while but we just retired 2 of our strongest characters, so we'll see how long we stay there.