Possible battle strategies: The clearing the party met the group in will make an excellent battle ground, as archers would have a very difficult time shooting through the trees. It has an even terrain, and good battle conditions for a fair fight. However the defenders are out numbered, and a fair fightmay not be their best option.
One of the guardians tells them of an out crop of boulders that creates a very effective valley with only one way in, and a very small entrance. They would be able to take their oponents on a few at a time. the drawbacks are that they would be trapped inside in a battle to the finish. there will be no chance for escape.
Other possibilities include; fighting a geurilla battle by attacking their flanks from the trees. Trying to make a run for it back to the caverns (dangerous, if all of the attackers are not killed they WILL tell of the caverns location). And a host of others your party will undoubtably come up with. The point is that it is never possible to cover all of the angles, and you should never try. Some of the best adventures come about when the party surprises you and you have to just wing it. Never however, let your party know that you were unprepared for their plan. Just roll and pretend that you meant for them to do that.