Frozen banshee's poison sensitivity will make the poisons more potent. she will not deal more upfront damage than kael but can deal more even if built with full tank stats. 

but would you give up on sandlashed for her so i can have 2 poisoners?

Since it's a counterattack team, yes, definitely FB instead of Kael, not in addition to him. 

You may also want to consider dropping Apothecary and going for a fully speed tuned team, with Valk's counterattack buffs always coming at the same point in the CB cycle and a designated stun target who tanks the stun every time (it's very difficult to do that when Apo is putting TM boosts and messing up the rotation). 

hmm maybe scyl  will fit in for a tank :) ill try that out! thanks

If you allready use Sandlasehd, maybe you could put in a champ with usefull buffs that get extended, like Mausoleum Mage. You would have increased def from him and shields from Valkyrie, your team should be able to tank a lot of damage from this.

i'd recommend to switch out Kael for frozen banshee ( much better damage ) and Apothecary for a champion that has attack down on their A1, stag knight's attack down is good but not reliable in CB. 

But what about the decrease deffence?

Stag Knights atk down is good enough. The stunning hit from CB is based on enemy hp only (according to Hellhades), so you don't need atk down when the CB does this attack. Stag Knight offers 2 turns atk down with a 3 turn cooldown, one missing turn for the stun-attack.