I've only played 2 games and sailed through them both (apart from nearly running out of hero cards on second one)
The Masterminds, and most of the Schemes, in the base game are quite easy to beat.
However the rule book includes rule variants that increase the difficulty somewhat.
Try using these if you find the game lacking in challenge.
Alternatively, pick up one or more of the expansions - they all include Masterminds and Schemes that are harder to beat.