Hi, this is me, from after I played the game for longer XD. I just wanted to say I have figured out some legion strategies, after plaiyng against other players who used legions extensivelly, and here is what I came up with:
A: On the Iron Age, Legions are actually pretty good against most "light" units( Meaning most Bronze Age units, plus maybe the Scythe Chariot, depending on how good your legions are versus how good your opponent Scythe Chariots are ). Most counters to the Legion are quite "heavy"( Centurions, Cataphracts, Elephants etc. ). Because of that, Legions actually make a solid combo with Priests. They particularly help Priests against Heavy Horse Archers as it becomes harder for the HHA to actually kill the Priests in time when the opponent has to constantly keep then away from the Legions. This strategy seems to be the intended strategy for the Choson.
A minor counter to this combo are Chariot Archers, who have high conversion resistance, are great at killing priests AND are fast enough to hit-and-run against the Legion. However, in maps without too much space to maneuver, it is possible to corner the Chariots and then overwhelm then with Legions. Sufficiently massed Minoan Composite Bowmans are another deadly counter to this strategy.
B: They are also a better choice then the Centurions for rushing or booming tactics. That because they are cheaper and faster, and effective against most Bronze Age units( Yes, even Cavalry without the Heavy and Cataphract upgrades ), so it is easier to make a large amount of then and overwhelm your opponent before he gets the chance to research the counters to the Legion, then it would be for the Centurion. The same also applies to the Short Swordsman in the Bronze Age, though Cavalry can be used for the same purpose.
C: Last but not least, a Roman Legion may not defeat a Centurion in one vs one, but TWO Roman Legions will, while the Legion costs less then half the Centurion does. Considering the Romans are not known for the power of their late game archers, and that makes the Legion actually viable as a anti-heavy infantry unit to use as the Romans. You will STILL need your Centurions against heavy cavalry and Elephants, though. So dont get too cocky.
EDIT: In a sillier note, for some reason the wiki tends to repeatedly mention the Centurion as having "Very high health" even though it is actually pretty average health-wise: Tied with Legions, and having less Health then a Cataphract without Nobility or a Cavalry with Nobility. It is not the Centurion-Health-That is its strong point, but its high hack armor and the fact it attacks harder and faster then any meele unit on this game.
[This message has been edited by Draco_Wolfgand (edited 08-19-2017 @ 11:47 AM).]