Missile spells have a radius and speed. Radius is how close a creature has to be to the projectile for an attack roll to take place. Speed is the distance the projectile moves each frame.
Mouse aiming shouldn’t be frustrating. The best way to aid this is to give projectiles a very wide radius. However, make the radius too wide and a stationary enemy might be attacked by a projectile twice on consecutive frames. This messes up accuracy as a stat.
One solution idea: each time a missile attacks an enemy, hit or miss, mark that enemy as Targeted. This enemy is no longer a valid target of this projectile. Targeted can wear off after a very short time (say, 5 frames). Because of the global cooldown (1 second) and because there are not multiple heroes, we can just set Targeted every time an enemy is attacked and do auto-miss if still targeted.
That way we can increase the radius of ranged spells without worrying about overlapping attack rolls. One side effect is that the Multishot skill is fixed and becomes an AOE skill as planned instead of a WeaponX3 skill.
One concern with this solution is firing a new missile while the previous missile is active. I believe this is not possible with current timing.