Siegecraft Commander

-- Disclaimer --
This achievement is really buggy if you are playing in Xbox One, i don't know if this an intentional bug from the developers to push the achievement community in xbox to purchase the game again in windows (as some users reported) or a legit bug they didn't care about back in the day. In any case, please don't buy it twice, supporting developers with bad practices or who don't give a shit about their players is something i will never recommend.

-- Alternative solution --
So you have been trying the top solution in local multiplayer, 20-min laggy split screen and all, but your reward refuses to unlock, i was in your belt, i know the feeling, but don't panic yet, one user here "Lakeshow" shared the real solution as a brief comment but the owner of the other guide didn't care about sharing in the main post, so i will just elaborate here.
What you need to do is to go to the first campaign (left one) and push it to level 3, it's really simple, the first level is just tutorial and the second you just need to build barracks to 15 men cap and let them do the job. Now, level 3 is where the magic happens:
You need to capture the gold mine at the beggining, then build 3 barracks and 3 spires (similar to multiplayer) but don't encapsulate the minions, let them be free so they can walk in-out and one by one, in the haste spell (x3), this spell is permanent (as far as i know) so you only need to launch it once. For a more graphic solution, your setup should be something similar to this:



Once you have this setup you just need to let the game run, the achievement will trigger at some point, for me it was minute 8 at first try.

I know this is a buggy achievement but following this solution should force it to pop (on Xbox One).

1) Build a barracks with your main account in an open area, then wrap walls around it. Make sure the walls can be destroyed without destroying the barracks.

2) Build 3 garrisons with the 1 barracks that will spawn units, if the wall was built correctly they will stand still.

3) Build a library with 3 spires near the other side where the dummy or boosting partner will be.

4)) Take the dummy or partner and build a bunch of barracks (without garrisons which spawn units) in a wall-like pattern so the units will bunch up when attacking.

5) Use the TNT on a dummy outpost to open the wall and release the units. When the units get near the dummy/partner base use haste 3 times covering the area.

6) Make sure to kill the grouped units with Dummy/partner after they destroy most of the buildings then quickly build more. Do not destroy and more units and do not use any more haste's. If the achievement does not unlock within 20-30 minutes repeat the process.

Good luck.

This one was a little glitchy for me, fair warning. The idea is to use the human's haste spell that they get from the shrine building to boost 8 of your soldiers at the same time This is easily done in multiplayer with two controllers or a buddy. You be the humans and the dummy can be whoever they want. The dummy account should build out a ton of outposts and libraries/temples to stall for time. Your main account should build a library and shrine close to the enemy outposts, followed by fourish garrisons and then fourish barracks to get a huge swarm of soldiers going. Wait until the soldiers gang up to hit a library, then launch a haste spell from the shrine at them to hopefully get the achievement. Make sure you have an outpost in a gold reservoir to be able to cast haste. I had to do this several times to get the achievement to pop. Good luck and happy hunting!