Symphony of War: The Nephilim Saga

On specific outdoor maps, look for a circus tent. Bring someone over to buy stuff.

The mercenary will be under the hire tab and will have a unique name

Completing the game on Warlord will give you Peace in our time (completing the game on captain)

You only need to defeat the empire reinforcements (all cannons and musket troops). Not the lone troops that don't move off the forts.

Make 2 squads that can survive cannons blasts at full strength.
2 paladins + hero + dragon + 2 healers
(I used Main hero & Diana)

Use every item you can on them to make them stronger.
XP/CP/Health/Proof of Merits/Traits/Best artifacts

Make as many extra squads as you can filled any extra mercenaries + clerics + archers.
Play on easiest difficulty.

When the battle starts,
Keep one newb squad on the castle in case.
Send one of each tank squad to both openings.
Send archers to the bottom wall. Keep your good archer squads out of the range of melee troops.
Keep everyone else away from the main battle but not too far.

When the reinforcements appear.
Keep your tanks at the front line to absorb as many hits. Keep them as far away as you can from the castle. Use the Caution attack.
Use the archers on the bottom wall to divert as many cannons. Attack using target leader (to help get rid of the cannon leader)
Anyone that rushes past your tanks should be taken care by your secondary squads.
The secondary squads should also try to intercept any cavalry going to attack your archers on the bottom wall.

Use the monastery as much as possible.

There is a turn limit. I believe it's 12 turns after the reinforcements appear.

You will lost most of your squads.

Make a quick save before you start the level.
Quick save before you start each round.

You need to force surrender most of the non star troops.
After surrendering a troop (with 3 left, the two bowman at the back and one on right wall), a text bubble opened up thanking me (and you get a ring).
Achievement should pop after that.

No need to complete the level.

Tips for the actual fight.
Get as many squads out.

Turn 1
Take out all troops outside the range of the main hero.
Try to take out the easy troops on the right side first.
Make one really weak squad and send it on the left. It should attract the attention of any easy troop you don't kill.

Turn 2
Clog up the middle with a tank squad (like Diane)
Clean up all other easy troops with flyers or archers. If you have the artifact that allows melee to attack up wall, it will help.

As the achievement says, you need to purchase all tech.

This is achieved by getting as many stars and upgrading your rank.
Do all the story conversations.
Get S rank in missions as often as you can.

Surrender as many troops as you can.
If your troops one shot enemies, just use the Force surrender ability often
There's a skull artifact that will help your troops surrender folks on regular attacks.

Arena fights don't give many stars but if you are in a bind, they help (Give about 10 stars)

If you overpower your foes all the time, you will not get this in a playthru. (I didn't on my warlord playthru)