How do you stack stats? I can't figure it out
As long as there is only one trait between father and mother, for example father being a goblin with +5 attack, and mother being a human with +2 attack, the child is going to have a trait that is the sum of his parents'. That is, it would be a goblin or a shortstack with +7 attack.
If it's a goblin, replace his father with him, so that his child would have +9 attack.
If it's a shortstack, take her to the main character, and breed her until she gives birth to a human woman with +7 attack. Then replace the +2 woman with +7 woman, which would greatly improve the speed at which your goblins improve. At that point, you will probably have a goblin with +9 or +11 attack, and his child with the new woman would be +18 attack.
Repeat until your shortstacks can one-shot an entire raid.
Now, you may feel your shortstacks aren't tough enough to take the hit. You can breed a completely separate breed of goblins that have bonus to hitpoints. But at some point, you might feel the meat shields aren't an efficient way of using your capacity. In that case, you can breed your woman (who at that point might have damage bonus around +100), with a meat-wall goblin (who at that point might have health of about +300). The result is not guaranteed, and it would be either a goblin or a shortstack, with stats which can be one of: of damage +100 and health of +300 (which is what you want), just damage +100, and just health of +300. Just keep breeding until you have a shortstack with both stats you want. By the way, replacing goblin with a new goblin with both stat bonuses would give you more damage, but would not improve the probability of having both stats.
Then you can breed your uber-shortstack with main character, with the goal of making more shortstacks like her. This has a much higher rate of giving birth to shortstacks, but not to getting both stats.
There is just one more thing to note: one of the posters before confused stat bonuses (which inherit) with stat base values (which are inherent to the type of being your units are). In your starting mansion, there are no units with stat bonuses. You will need to, instead, raid either northern human village or one of the monster lairs. There you can find people and unborn monsters with stat modifiers, which you can later use for breeding.
It's much easier if you start with a woman with stat bonuses (otherwise, you'll need to wait for a shortstack or an oni with bonuses to be born to give birth to woman with stat bonuses), but attacking human settlements will increase your infamy much faster, which can lead to trouble with defending.