The old campfire levitation trick still works. Put a campfire down and then place a board underneath it (you can square if off first with boards and then place it). As long as some of the flames still rise through the wood, it will stay lit.

Then you can pick to excavate all around it. As you dig around, make sure to use vertical wooden beams to support the floorboard. As long as you keep the floorboard supported while excavating, the campfire will remain levitated.

A quicker way is to get a bunch of stone and raise ground with the hoe. Raise ground directly below your feet for as high as you wish to. Drop the campfire on top of it, and then proceed with the floorboard beneath and supporting beams.

After either of these methods you can even remove the floorboard and the campfire will remain levitating as long as there are those supporting vertical beams in place, or something else that's load bearing. Try experimenting with it. Any piece not dark red on load bearing can support the campfire after removing the floorboard beneath it.