Tips for double battles

I’m trying to complete a nuzlocke and the only battles that ever give me trouble are double battles. (Serena in particular). I change the terrain but still find myself losing. Does anyone have any tips to get better at them in general or have a link to a good guide to help get me better at them?

(Ps I’m switching off between doing a randomized nuzlocke and regular dexnav nuzlocke just to keep things interesting. I’ve made it to the elite 4 twice in quite a few tries but have never beaten it and am fairly experienced in pokemon but I’m playing without EVs