Just a couple of things to note:

• Every Mass Combat Action aside from Rally has the Movement subtype. Not an issue per se, but it makes the water opportunity "That leader’s cohort cannot perform Movement actions until the beginning of your next turn." really powerful.
• Mass Combat objectives do not scale well with the number of leaders.
  • By a literal reading, if one cohort Digs In and is Assaulted by 3 enemy cohorts, one doing 8 casualties, one doing 8 casualties and one doing 4 casualties, then the fact that one of those cohorts caused less than 5 casualties is enough to complete the objective.
  • By comparison, Grind Them Down requires you to cause 10 casualties in a single turn, which with (for example) 2 leaders in a small party is nigh unachievable, let alone a solo player.
• Having the objectives state a casualty limit as a multiple of the number of leaders (in the same way the Discord Wheel roll scales, or Intrigue Objectives scale with the Vigilance of the persuadee) might help reign this in.
• Panic does not seem to matter very much.
  • Assuming you lose a Grind Them Down objective every round, you'll be suffering 5 panic, plus maybe 1-2 panic from the odd fire opportunity.
  • A samurai army can tolerate 60 panic, and any time you spend the odd water opportunity to remove some, it'll be 5 or so points (because your glory rank adds under the army rule).
  • Even if you don't remove panic, that's about 10 turns to 'break' the army - and ten turns of losing Grind Them Down objectives (which requires 10 casualties in a single turn) would have been enough to exterminate the strength 55 army twice over.
  • Yes, battle fatigue matters. Failing a TN4 check is easy, and 10 panic a turn matters. But that's still likely to take 4-5 more turns to break the army, especially if the commanders start making efforts to keep the minions in line.
  • There is no action you can choose to deliver sizeable chunks of panic to the enemy. Samurai Archers performing Reinforce (Fire) is the only one I can see off-hand.