The way Graboids hunt is Vibrations through the ground. even in the Trees the Graboids would know which tree they were on. Not only that the Predators cant SEE them under the ground to begin with until they pop up.
I wouldnt be surprised if one Predator got eatten here tbh.
I thought that a Predator could use his staff to just spear into the ground but since they cant see the Graboids heat signature then that wouldnt work either.
As far as we know from the movies the Predators have three visions. Heat vision, the green vision to see the aliens signature(Not sure what to call that.), and then the white one that seems to be able to track lights. If I am missing one or two update me. Even with these Visions they still cant see through solid objects, IOW the ground.
Graboids tend to surprise people by reacting right away to vibrations. Now to be fair a Graboid wouldnt just take a tree down in an instant. They would have to dig around the Tree roots in order to knock it down. Predators could use this time, knowing the Graboids are under the certain tree, fire upon that area.
As for using their explosive wrist device that would only work under the ground and even then might even take out the predators but doutful it would go into the ground enough to kill the rest of the Graboids.
Now IF it was the different stages of graboids, The little flying burrowers, the Graboids, then the heatseeker Graboids, the Predators would lose. But up against normal Graboids it is hard to tell.
IMO, If the Predators do happen to win I dout they would be Flawless victory.