U said tanks? In Birthright game, u cant recruit any Knights. So during a mission, when you have Orochi on your team, keep in mind she has the Capture Skill (Capture an enemy unit, to bribe/recruit later in your castle). When you pass by a Knight with good stats, make sure Orochi can finish the Knight off. Instead of Killing him, Select Capture. Capture the Knight, then grind-train the Knight from Challenges, and any DLC missions you bought. (DLC missions are replayable) I havent played all the DLC missions, but from what I know, Boo Camp is great to help train and level up your characters.
P.S. (my own personal opinion, you dont have to listen to it) Dont bother capturing more than 1 character, because
1. They have no Support Connections
2. Having more than 1 would steal away exp gain-leveling, compared to the captured character being an actual, REAL storyline-based/support character
On the bright side capturing helps get you get classes you dont have in your team, maybe a bit useful for those playing lunatic to help fill in slots for characters that have died throughout the storyline.