On most scenes, if you failed the mission or retreat, the scene will start over. However, your experience points and level remain the same. Use this technique to gain experience points, then try to start the scene over again and keep destroying LEVs. Do the same thing over again as many times as needed to gain a large amount of experience points.
If you want to get the best attack for your OFs and Levs, level up past 45. Some of the attacks are 155mm: Cage; Geyser: Twede; Final Blow: Myona.