Okay after testing this with many of my castles I have came to a conclusion. The link they grow VARIES on many factors that take place when they train. These factors are:
- If the Pokemon survived in the simulation.(NOTE: This is mainly a speculation as I cant see if they actually live through the battle without fainting, but I noticed that sometimes they would gain more from the same kind of battles then other times so I just assumed that sometimes they fainted other times they didnt.)
- What kind of Pokemon they will be fighting. For example you would get more from a battle with more powerful Pokemon like Aggron, then from say a Aron.
- If they are fighting Pokemon that are in the wild or if they are fighting a Pokemon that belongs to a warrior. As you may know you gain way more Link from battles with castles than when grinding inside a castle, its because you are fighting against other warriors instead of the normal wild ones.
- If the partner the warrior is fighting with has a better link capacity they tend to gain link faster with said Pokemon than with another
If I find more factors I will be sure to let you in on the secret.
Source-Series of multiple castles test on my own game.