I haven't DMed a Pathfinder game in ages.
When I did, I recall my usual method being to keep the easy fights coming as usual, and occasionally (like, twice a session? More if the sessions are particularly long) throw out stuff intentionally built to kill them effortlessly. This stuff would be named enemies, setting up a foe and a plotline piece by piece. After every encounter they "lose", weaken the next a bit more. The strongest one is final boss of that plot, next strongest is his right hand man, etc. etc. Then instead of killing them when they lose an encounter, they get beaten soundly, mocked, and the enemy would get out before they got back up.
Sets up a story, fleshes out the world, and lets you test what their party is capable of.
Meanwhile keep raising the "floor" difficulty, e.g. easy fights, steadily. By the time they beat one of your "named" foes, you should have nailed a comfortable medium.
It's not the best method, but I never was very good at nailing difficulty with munchkin players. I did best with players who wanted a cool character that maybe ended up a bit shit.
preview edit: And of course, Roger is absolutely right. An easy campaign isn't bad if everyone enjoys themselves.