I am running a game for some friends. I intend this game to be casual and light hearted, and I encourage unusual builds. As such, all four party members built martials. We have a Barbarian, a Sniper Slayer, a Swashbuckler, and a Paladin. Everyone is having fun as far as I can tell, but a predictable problem has emerged.
In a straight up fight, the party annihilates even tough foes, to the point I worry they might get bored. The one challenge they faced was a swarm, which hurt them badly, and they could barely touch. I want them to be challenged, but I worry that, for example, incorporeals will either TPK or cause retreats. I also worry that they will soon get frustrated with either easy battles or ones they can barely touch.
What are some good ways to handle parties with such might, and such obvious flaws?