Post your Mechs on guard mode on top of hills. They have the ability to see the enemy coming earlier than normal and have a distance advantage. Also, any weapons fired will come down on top of the enemy (which is dangerous for Mechs since their head cannot take a lot of the damage, which means the pilot could be killed). With that in mind, do not put yourself in the low valley position, unless you are sure there are no surprises on the hilltops ahead. The one advantage of being down in the hills and valleys is that you can evade enemy radar at the same level as you.