The feat Corellon's Wrath Style DDI will upgrade riposte strike to deal extra Int damage to demons, though that is a light blade attack not a crossbow attack.
The feat Demonbane DDI allows divine attacks that target undead to also target creatures with the elemental origin (which includes demons). If you multi-class to Cleric using the Channel of Faith multi class feat DDI you gain the turn undead power. Likewise Channel of Vengence multi class feat DDI will get you the Avenger variant.
Demonbane weaponsDDI grant you
Property: On a critical hit you deal +2d10 damage instead of +2d8 against demons.
Power (Daily): Free Action. Use this power when you attack a demon with this weapon. Gain a +5 power bonus to the attack roll and ignore any resist value the demon has.
Demon slayer weaponsDDI grant you
Property: On a critical hit you deal +2d12 damage instead of +2d8 against demons.
Power (At-Will): Free Action. Use this power when you hit a creature that has the demon keyword with this weapon. The demon does not benefit from variable resistance until the start of your next turn.
Power (Daily): Minor Action. Your next attack with this weapon against a demon, if made before the end of your next turn, gains a +5 power bonus to the attack roll.