The main goal to have strong units is to have generations of children. To complete this you need to level your units to maximum level then those units will mate and have strong children. Then you level those children to maximum then mate them to have another breed of stronger children. That's the cycle of units. Also when units are maxed level, they can be sacrificed and turn to crystals that can be used to improve your units. Be aware that there are maximum status for units depending on their job classes. So there is a limit to this "strongness". Refer to this by zsaber -> Max stats of Jobs

Things you need to know about mating/birthing/breeding:
1.) When you place a couple in warrior house they will or will not show a heart symbol. The bigger the heart the higher the chance they will give birth. If there is no heart they will not give birth.
2.) Age of both couple must be 15 or above.
3.) Mating finishes every "day 1 month 1". So every new year in-game, that's the point a child is born but there are some instances of bad luck that they will not. Also technically speaking, a couple can give birth in 1 day if you placed them in warrior house at "day 31 month 12" and pass 1 day.
4.) Each unit can bear a maximum of 3 kids whether they mate with different partner or not. The chance they give birth are lessen every time they mate.
5.) Units from both either parents can't mate. So technically speaking, "brother and sister" or "step-brother and step-sister" cannot mate. Be reminded! Same gender can't bear kids!
6.) Child will inherit random amount of stats depending on the totals of each stats of both parents combined then divided randomly. The higher the stats of the parents, the higher the range of the random scaling of inheritance.
7.) Child will inherit 2 skills randomly from both parents.
8.) Child has a chance to inherit the blood of one of the parents.
9.) Child will be born as a random class from one of the parent's job class. Example: Dad is a Highlander and Mom is a Spear Master then their child can be born randomly as a Sword Fighter, Spear Fighter, Spear Master, or a Highlander. But the higher the job class the less chance he will become one but still possible.

You will need luck and some other info to bear strong children.
These are:
1.) Bloods are a good way of increasing the stats of units. Each Blood has a unique ability to increase certain stats. Sometimes units with blood appear in square by chance. You can also sacrifice your blooded units and have a chance to create a crytal that can give blood to a unit through that process.
2.) Another form of stat boost is upgrading your unit's job class, since higher jobs gain more stats. It is best to upgrade your unit's job at minimum levels preferably level 1, so that they can take the full benefits of the upgraded job. But there are some instances that you should not upgrade them early. Most reason for this would be trying to gain a maximum skill level. Every job class can't level all skill types to maximum but each job have their own ability they can level to max. Refer to this -> Job Learning and Master skills
3.) Parents' upgraded job class also plays a role in bearing a high-stat child. Every job class has its maximum stats at birth so higher job classes have higher max stat for a child to store. Refer to this by zsaber -> Max stat of new born baby
4.) When you look at the guild window, there's a line of numbers called Tribute Points. The higher the tribute points the better chance of: the child gaining more stats from parents, inheriting a parent's blood, and being born as a higher job class. Tribute Points is the total effort of the guild. It is increased whenever any members of the guild do the following:
*raid other players successfully
*one of their units die or sacrificed
*they use holy devotional stones
The fastest way for a guild to gain tribute points is to capture a fort(not referring to forts in base but the castle-like forts in colonial war). The guild gains 200 points for each fort and they can capture a maximum of 2 forts.
There are things I might have missed though. Anyway tribute points is really helpful in mating. Everyone in the guild will benefit from this points. Tribute points are reset to 0 every maintenance. Also it is after maintenance that guilds gain tribute points from captured forts.

Also you might want to check out some guides and tables here on how Einherjar works.
For example, stats in einherjar are gained by chance through percentage.
Here is a link for this info by zsaber.
Job, Blood Increase Rate
This is a very helpful table.
If there is something that I missed please tell me.
This is for a sole intention of giving our new players the knowledge for them to know about the game, give them interest, and expand our community. I hope this helps.

by nerovesper

Source: http://gameeinherjar.wordpress.com/2012/08/22/generation-of-strong-children/