In addition to what N said, don't waste items on characters you know you'll never use. If you plan to use classes with three weapon choices (eg. great knight), the arms scroll will benefit 'em much more than other characters. Remember, you're not going to have "challenges" like in Birthright, so leveling your weapon proficiency is going to be quiet the task.
In my playthrough, the only way I got to make Mozu a sniper and leveled up her weapon proficiency to C (so far, but I'm aiming for A) is because I used the Boo Camp DLC for EXP. Without DLC, you could never do that late-game.
Try to avoid losing any of your royals as well. They'll be helpful in the long-run, including Elise (she has a dangerous magic stat).