I've played games my entire adult life and I run a game club for high school students. I am also a teacher and so I have a lot of experience with this situation!
Playing a couple "open" games has always been the most successful teaching tool that I have found for learning new games. An "open" game can be defined as a game that is openly discussed as it progresses. So, for example, as you take your turn, tell your friend some examples of what you COULD do and then tell your friend what you WILL do. WHY did you choose that choice from amongst the others? When it is your friend's turn, let them execute and make choices, but gauge when it may be appropriate to join in with "warnings" when they are about to make a big mistake and "congratulations" when they make a good move. Gamers can be sensitive to people "helping" them, so gauge what will work with what you know about your friend. The goal is to help them get better, not to play the game for them.
Another alternative, if you'd like to keep the games "closed" is to have a "debrief" after every game you play. What did they do well and not do so well? What did you do well and not so well? Admit when you make mistakes and ask their opinion on what they think you should have done in a tight spot. You may end up improving from this dialogue as well.
What both of these techniques have in common in that you need to prioritize the learning over the "winning." If your friend's improvement is important to you so you can have a more challenging opponent, you will have to be willing to create artificial situations that allow your friend to learn by experiences, in a controlled environment in which you can help their learning happen faster. There is no substitute for experience, but GUIDED experience is even more valuable.
Finally, this game has a lot of rules and the cause of some mistakes may simply be due to too much complexity too early. Start with small lances of one to two mechs with simpler terrain and work up. Perhaps you have a game with just hills and then a game with just trees and then combine them. Add rules until you are playing as deep as you would like without making mistakes.
I hope this helps! :)