Pillars of flame
Step one: collect flammable material from the edge of the jungle/brush from the mountain. Preference should be given to things that cause a LOT of smoke and burn slowly (Can be avoided if the pyromancer is the "can make flames appear and burn without fuel" type) This should essentially be what everyone not the pyromancer/earthbender is involved in for the entirety of the 12 hour period. The pyromancer should be resting, they have a busy night ahead
Step two: While the fuel is being gathered, Earthbender should create a 15ft circular wall around a space capable of fitting your entire party and the fuel they've gathered comfortably. The wall doesn't need to be particularly thick, it's there to mask your party from view of whatever megafauna might be around. Ideally they just raise up a circle of 3 inch thick stone or something.
Step three: Earthbender creates a ditch in front of the wall, 10+ft deep, wide enough that nothing can jump from the far side to the wall itself. The depth should really be as deep as the earthbender can manage, so when the rest is set up they should spend the rest of their time deepening/widening the ditch.
Step four: Raise up pillars of stone/earth 12ft tall, with a slight depression at the top (think a shaft with a bowl dug into it). they need to have a cross section of a couple feet. Top them off with piles of wood/other fuel. 548
You want these every yard or so.
Step 5: At dusk/when owlbears appear, light the fires on top of the pillars. The pyromancer's job is to keep those things burning all night. Depending on your cast of characters and the power of your earthbender, you might have shifts which climb makeshift ladders to pile more fuel on, the earthbender might lower low-burning pillars to add fuel, or raise up temporary "steps" so they can be refilled.
What does this get you? The wall/ditch should obscure you from view, and be enough of a weirdness to keep the larger predators away. The smoke will mask your party's scent. The combination of fire and smoke should also keep the owlbears from wanting anything to do with you. If your party does things particularly well, the walls being several feet higher than the fire-pillars will also stop the flames from being seen from a distance, which also helps keep away the curious come nighttime. When dawn breaks the earthbender lowers a patch of wall, fills in the ditch, and off you go!
Sidenote: If your walls need to be of any serious thickness (several feet) then you'd also want some spiky vines, a "step" with more fires on the inward side, or worst-case fires around the top to prevent the owlbears from landing on the wall and then slipping down the inside in the night. Mind you this whole thing is overkill on my part, you could easily get by with a ditch, three fires around the perimeter, and a few sharp sticks. But as they say there ain't no kill like overkill! 548