Strong against: Armies with low power and/or little in the way of commands
Weak Against: Armies with units that do not 'kill' units, but typically remove them from play for an extended duration, if not for the entire game(e.g., geomancer/witch), higher level AI.
Best Paired with: High armor, high HP units, heretic.
While its other abilities are certainly nothing to spit on, the FS ability to raise army-wide reduction by one is, insofar as I am concern, its most potent one. It is the only way any unit can reduce more than 2 incoming damage. Most units in this game do 3-4 damage; these units now make no, or a very small, dent in your already two armor units(footman, warrior, etc). Very few have five or more power, and these tend to be clumsy and expensive, so that it's hard to land multiple powerful attacks on units that now have an effective three armor.
Some things to remember about fighting a feathered serpent army:
-If you didn't kill a unit, you might as well have not damaged it at all. It's getting a full heal next round.
-If you didn't kill at least two units, your attack was only a minor setback for your opponent, as he can just put the dead unit back on the field. On the other hand, you probably exposed your attacking units to a great deal of harm-and you can't get them back.
-It takes quite a few commands to successfully take out two mitigation-buffed units.
-DON'T FORGET that all of your opponent's organic units reduce one extra point of damage per attack.
FS armies have one weakness: they probably don't have that many pieces on the board, since the FS cost 550 points. This means that you can often maneuver around your opponent's pieces to knock out the FS(warning-the FS is a slippery unit)
Otherwise, you will likely have to slowly advance up the field in a conservative offensive formation: acolytes, units with dodges, masons, and/or healers are necessary to make this happen. Guard witches, geomancers and the like closely; they are your greatest asset.