It's gonna take a long time if you want to have a strong unit.
+ First, you need to have some soldier max at least 1 status, this will give you crystal to add to your favourite soldier later. Low job soldier will get max stat easier than higher job.
+ Second, BIRTH, this is very important step to get a powerful warrior. The new born warrior will inherit the parents' strong point and surpass them. There're 2 things you should notice: let soldiers max lv 45 mate each other will give you a stronger baby, you should mate different types of warrior to make up their weak point for the baby. Eg: I always mate Axe and Sword so I can have a warrior with high str and tech, no more worries of missing.
+ Third, you can get special units from quest, though they're not easy to get, but if you get them, make them bear all 3 babies, from my experiences, when you mate a normal with a special unit, their babies will be a lot more powerful.

(Author: zsaber)

Source: Einherjar | The Viking's Blood • View topic - Max stats of almost jobs