In Dokkan all characters have Links. And with a recent feature added to the game, Characters can now get aquire levels for Links powering them up.
Fully leveling a card to 10 will take a VERY long time it can take 1000s of stamina in order to fully level a card, as you get higher up in link levels, the levels become harder to get. If you think it gonna grind all day for a full link level 10 you are mistaken
How are Links leveled
Once you enter a Gameboard every fight on it is secretly given a cards link level. So obviously you want to hit as many fights in a single level as possible in order for a chance at more level links. Fights can also have a higher chance at leveling due to some less than perfectly known modifiers but difficulty seems to be a major one.
What about Transforming Cards
Transforming Cards are basically counted as "sperate cards" so you'll just have a chance for their exclusive links level like any other link.
So if a card with 5 links transforms and has one link change he technically has 6 links for you to level
Best Stages
Manual
Area 15 Stage 2 - more consistent
Area 23 Stage 8 - higher potential
Area 28 Stage 6 - new hotness
Auto
Area 31 Stage 4
Boost does effect the rate at which links well level but only because boost literally just lets you run an event twice once, it's merely a time saver