Need stocking farming help

So my farming team is Nitocris (5/6/1)/Scathassasin (1/1/10)/support waver/ ridertoki (1/10/1) All bonus CE
I finish the level in about 6-7 turns which is taking too long.

I start the first wave by using Nitocris second skill NP battery and first skill insta death to clear the wave. But instadeath still only procs about half the time, usually the robot and reindeer alive, so I have to waste a turn to clear them. Then second wave, plugsuit kintoki in for Nitocris. Kintoki NP battery and quick buff, waver skills on scathach. I used Kintoki NP at first card slot and try to do quick chain with scathach or kintoki brave chain with quick card in the third slot(kintoki NP leaves dragon with about 5-6k hp). Then on third wave, I use Scathach quick buff and fire her NP( If I’m unlucky, I have to spend another turn to charge her NP). However, her NP doesn’t kill ceaser and Santa alter. I have to waste two turns to kill both of them(or if I’m lucky and crit).

Can anyone suggest anything I can do to improve the farming efficiency? Should I switch out Scathach CE for more offensive CE instead of event bonus? Would it be worthy investment to upgrade Nitocris skill 1 for more instadeath chance just for this event? How high does Scathach ATKhave to be to kill ceaser on NP?