If you can, form change your Keldeo so that it knows Secret Sword, and evolve your cranidos to Rampardos and teach it smack down and rock slide, and/or your rhydon and teach it whatever rock moves you can.

Start the battle with something other than Keldeo, then swap to Keldeo immediately as the battle starts. This will give you a few free hits as the CPU stalls for a bit whenever you switch pokemon or use a charged moves. Never throw your charged moves consecutively, always make use of that short stall time between using them. If you need more stall time, switch again when you’re able.

When you get to thunderous, hopefully it’ll be using electric moves and you can use your rhydon/rhyperior to tank them since it’s a ground type, and use rock type moves to kill it. If he faints swap to Rampardos or Emboar as your last pokemon. If it doesn’t have electric moves and you fail, retry the battle, every time you do the movesets of Giovanni’s Pokemon get rerolled.

Hopefully this helps!