Board Thread:Question and Answer/@comment-24.207.136.202-20161202224351/@comment-8642280-20161204024925

Assists by other players are from your comrades list. The more recently logged in they are, the higher chance the comrade will come in at Battle Start for the assist. So it is random.

To assist another player, simply refresh your game, and for another player there is a chance that you will assist in someone else's battle.

The chosen card for the assist is typically what strongest cards you have by default ordering of cards at "Edit Unit" and "Upgrade" areas in the 1st to 2nd page.