Battle 3:

Mikhail (4000) vs the Pupenheim Mercenaries (4000, Gush Wind)

The mercenaries are very easy to defeat if you use Backwater.  Backwater is an exceedingly slow formation but has high attack power.  Basically, keep moving forward while the mercenaries alternate between using Speed Atk Fwd and Retreat Atk.  When they use All Charge, the defend until their morale reaches orange.  Use All Charge, and your army will walk straight through them.  It's as easy as that.

Cowards...

If done correctly, this is what you should see after they've pulled their All Charge.

I recommend trying to grab the Gushwind Formation from this battle if you haven't already.  Grab Strong Atk as your second tactic if at all possible.