You cannot assign R2-D2 to Biggs Darklighter's X-Wing Fighter, he was with Luke Skywalker for the entire battle of Yavin Four survival. R2-D2 wasn't the only Astromech Droid in Red Group when they attacked the Death Star in Episode Four A New Hope. There were a multitude of Astromech droids for each and every fighter that was attacking the First Death Star. There was no way he could be in two places (X-Wing Fighters) at once. You guys really need to fix this canonical error in your final tutorial for the miniatures game.
An Irritated Fan