For me Toxic worked for every single run I've had against Terra however if you didn't get a pokemon that had Toxic before leaving the city then this doesn't work since Amethyst made it so that you need to beat Terra to get another pokemon which can learn Toxic. However Toxic Spikes should do because although she made it utterly impossible to find pokemon with Toxic if you didn't go out of your way to get them she made it possible to get Toxic Spikes, I'd recommend temporarily replacing Manectric since it's pretty much dead weight for your team and using Qwilfish which you can fish up with the Good Rod and it can learn Toxic Spikes so just knock out the Nidoking then set up the Spikes and that should deal with all of her team except Excadrill which you can use Charizard or Sharpedo to deal with then you can use Cotton Candies to stall Terra out. Although this doesn't work if you didn't pick up the Good Rod.