1d12 + 12 is causing you problems?
That is a really reasonable amount. That's a 18 Str character power attacking at level 6. (+6 Str +6 Power attack)
Even at level 4 that isn't very far out of reach. I have no idea how he is getting a +6 against demons, but frankly I don't see how 18.5 damage per round is too much. Is the cleric really overshadowing your combats or is he just a heavy hitter instead of a dedicated healer?
As to how to challenge your party:
Flying creatures
The three players you have issue with are all melee. Have a few winged archers and they will hate them.
In Sword of Valor
Spell Casters
Spell casters tear through melee types. There are a bunch of juicy ones coming up, don't be too afraid to use them to their fullest. Also remember, they can run and fight again after their meat shields die.
Terrain
Your PCs are mostly melee, it isn't your job to make sure the party can easily fight the enemies. If they can stay behind an arrow slit, they should. PCs don't like being told they can't attack, but they like having to solve a problem. Make sure you toss an encounter or two their way where they can use terrain to their advantage.