I also face swarm armies commonly and i usualy take farsight and seven bodyguards all with 2 gun drones and two flamers. This supplies your crisis with a defense of 14 drone wounds and flamers, 14 of them deals a lot of wounds epecialy when your opponent cant make a save against them.
Since you don't have the units to do this, as you said stick your ethereal in the centre of your fire base but put the ethereal in the devilfish. Put one unit of kroot strung out in front of your firebase. When the orks come within 20" use invocation of fire this and next turn. This will give you 3 shots from all your kroot and fire warriors. You shoot the ork hordes back far enough to stop them charging (as you have killed enough of the front to reduce how close they are) Continue this until they can eventualt charge you. At this point they will have to charge the kroot as they are being a meatshield for your fire warriors. As everything is within 6 inches they can all overwatch. 8 markerlights, on average one markerlight on. (Do pathfinder overwatch first) Now one of your fire warrior groups can overwatch on 5+ instead. If the kroot survive the charge then deploy your other unit of kroot as far in front as possible and move your firebase back 6 inches. Then you have the same formation as before so you can repeat the whole plan.
While doing this your crisis can jump in and out of terrain killing as many orks as possible, only after killing the trukk though. Use your piranas to grab objectives or come behind the ork horde and shoot at them hoping they will charge the piranas insead of the kroot (So you have another turn of shooting at them)