Okay, fucking ancient legionaries and honor guards. Just met eight of them. Lost four bros. Four! And I thought I was doing everything right, more or less -- shields, hammers, reach axes, the works. Damn thing is a steamroller. I'm feeling for those poor Germanians. Any tips?
They can be a challange... Wait until you meet honor guards haha.
The basic tips are always the same though. Don't attack certain specific enemy types until you have X armor on your guys and 2-handers in the front line.
One-handed weapons are imo very situational later on due to the fatigue cost, esp. with shield and special attacks. Spear wall doesn't work against them (except shieldless auxiliaries) because of super low damage and their ability to raise shield and then march in.
Greathammers can also destroy shields btw, don't need axes except maybe against knights/orc warriors. Warriors don't have enough defense to warrant destroying their super durable shields though.
Legion are mostly a problem because of their polearms in the back line and shields. Due to backline, you may not have time to break shields or kill the front line They don't have much armor or HP except maybe honor guard.
So what you can do (if you don't have warscythes) is: Give polearms to your front line or greatswords (greatswords aren't as good against orc warriors and knights though). That way you can hit the more dangerous bladed pike etc guys.
If you gave your frontline polearms (not greatswords):
Position yourself so their frontline ends up one tile away from you after moving (they aren't necessarily so dumb as to do that though). Attack their frontline with your frontline at the start of the turn (three guys on one target if necessary, you want to tear holes in the line). Then move your entire force back one tile so that there are two tiles between you and them, that way as mentioned above, their polearms can't attack you next turn. Gives you two turns of working on their frontline. Not advised against the better armored honor guard maybe.
You can also give your backline throwing axes, but they suck without the right perks imo. If you can spare a perk point on your ranged dudes (I never could), maybe you can go the throwing spec route in addition the bow or crossbow. But well, even with the targeting perk backlines can be hard to hit.
Other than that, high melee skill and just mow down their frontline...
Armor 210 minimum head and body imo for legionaries. Better more. These dudes aren't necessarily worth fighting anyway unless you're strong enough. Their weapons have low durability and can very much break in battle and leave you without weapon if you use them (happened to me with their spears using spearwall due to so many hits). The pikes and such sell for a decent amount, but so does other loot. Their armor costs too much fatigue for what little it does. Wish there were restored versions, or that blacksmiths or smth. could restore them for you.