Flank is when you put out soldiers on different planes to avoid running into strong units with heavy recharge time, this tactic can easily fall prey to spamming soldiers in a straight line because it can deliver territorial damage quick.
Strong-arm is to stop a pierce (or a butter knife) by sending in heavy cavalry to break through a pierce by having them kill fodder soldiers, they normally won't break through a pierce from a stronger enemy, but the main goal is to build up your charge. However, this tactic can easily be flanked.
Pierce is to spam weak fodder soldiers in a line, when they bunch up they get more powerful, piercing through the weak lines of the flank, but they are easily crushed when the enemy sends out very powerful soldiers.
Pierce > Flank > Strong-arm > Pierce
Pierce; fodder in a line... Flank; fodder on all rows... Strong-arm; break up a line with high attack power soldiers and build charge.