To answer your questions, elemental magic can kind of effect them. Elemental magic effects the environment, so if you created an earth wall, the monster could still make contact with it. However, due to the wall's magical origins, the beast would't have much trouble smashing through it.
And you wouldn't be able to effect them directly with your magic either. So you can't freeze their legs because that's a direct attack on them. Only way your magic will help you is by effecting the environment. Example: Using lightning magic to destroy a rock base, and having those rocks create a rockslide on top of the beast.
Also! It is true we already have 5 people, but I can make an exception possibly if we feel 5 won't cut it.