I found that dual-weilding works incredibly well with Deflection, everytime you make a deflection attack you also make an offhand attack. Etheryn is probably a liability here, her skills generate a lot of threat which means Deflection becomes less useful as more attacks are drawn towards Ryn and away from you.
While I've used the team of Me, Cait, and Ryn for a lot of the game I suggest you bring Vivianne. Once I actually read what her powers did I realized she makes for an excellent controller. Elemental Fury buffs you, buffs your powers, and buffs you Deflection attacks. If you're dual-weilding, you get double the value. Immolation debuffs enemy attack and spellpower, and the burning DoT is nice. Heat Weapon can force a disarm, and Summon Wolf Spirit gives you better action economy and can inflict bleed.
With Cait and Vivianne, open the fight with Bull Rush to take a turn from Cedric, Spirit Veil on you to boost evasion, and Elemental Fury on you to boost damage. Next turn, you focus on damage dealing, Cait can Heal or White Fire, and Viv can either Summon Wolf or Immolation for attrition, or Heat Metal to disarm Cedric. With Deflection, dual-weild, and high evasion, you can be making six attacks in a round. For an extra push, drinking some Cunning boosting alcohol before the fight for better crit chance.
If you get evasion boosting gear, the Kunoichi Set works well for this, then Cait doesn't need to focus as much on healing you. You can also stack Lucky Strike rings for better evasion, critical chance, and accuracy. The Heretic Triangle (DPS, Support, Controller [Holy Triangle is Tank, DPS, Support]) beats nearly the entire game. I think I could have beaten the black knight if she didn't knock out Cait with double crits in the first turn.