The safest and most legal way to acquire an ISO is to rip it from a physical retail disc using a homebrewed Nintendo Wii or GameCube console.
This is a defining mechanic of this entry. By completing side objectives (e.g., finishing a map within a certain number of turns, keeping allies safe), you earn BEXP. This can be used at the base to level up units in a controlled manner, allowing you to boost weaker characters. Skill System fire emblem souen no kiseki iso