You'll find a great deal of additional units as you go, so the ones that you do not require, use them for all the numerous approaches of improvement for your characters. You can compromise unwanted units for either improvements or upgrades. Upgrades simply level them up while improvements enhance all their base stats. The simplest and most affordable way to level up your units, however, is merely to replay levels that you have actually currently beaten, and beat them once more for the experience points.