I saw this question come up in world chat the other day. I happened to post on the formula for the mystic cave but the math will also work for plundering. BUT REMEMBER that the formula doesn't factor research bonus's, crit rate and any other factors. The formula's for that would be much more intricate and very hard to accurately calculate. BUT here's the answer to "how many archers will it take to kill one swordsman?"
Let's start with the basic formula
(Number of Units) x (Units Power) x 2 = Total Power of your forces
(Swordsman) x (Swordsman Power) x 2 = Power of Archers Needed
SO lets plug in
Swordsman = 1
Swordsman Power = 20
(1) x (20) x 2 = Power of Archers Needed
40 = Power of Archers Needed
Archers have 94 Power so you only need 1 Archer to kill 1 Swordsman.
The thing is that it doesn't always seem to work. I don't know the math behind it but if anyone has insight on how it works PLEASE shoot me an e-mail or message me at GGk723@gmail.com