Age of Empires II: Definitive Edition

This is a really easy achievement: you can have the enemy open the gates for you.

1. Play on standard difficulty. Roman Army seem to mostly ignore you outside of your base and there aren't any Onagers.

2. At the start buy all infantry upgrades from the blacksmith. Trade all wood for gold, using some of it to buy wood (keep doing this throughout until inevitably your market gets destroyed). Buy the universal upgrades and the one for long swordsman (even though we won't be training anymore after we get the ability to make Huskrals at barracks, you may as well make them useful for now.

3. Make sure you have 60 Huskrals (first one might not have enough money but that's ok) send them to the gate and start attacking it.

4. Once an enemy comes to confront you, opening the gate click to move them to the space behind. If there are only archer/Skirmishers then just keep moving away slightly until they are aiming at you whilst keeping the gate up. Repeat for multiple gates.

5. March straight to the town centre, don't bother trying to kill every last unit.

6. Rinse and repeat. On the last Roman Army (purple) one you need to destroy the castle, military buildings as well as the town centre but your Huskrals will rip through. This one most likely will require another 60 but still no trouble same as above.

NOTE: you will notice that as you progress from town to town the Roman Ay (purple) army will destroy your buildings. It's not worth making an effort to save any of them. The only exception is the Castle at the end (you can let the barracks be destroyed). You should have a second wave of units in the process of being created so you will be able to stop this.

Despite only training infantry units (the game specifies petards as infantry), it did not unlock for me. Keep in mind that petards may not count for this achievement. Apart from that, it should not be too hard, just tedious.

Remember to save frequently.

Upgrade all your infantry techs in the blacksmith and barracks. Speed and arson are good. Sell all your wood slowly because the price seems to improve quickly in this mission. Buy enough food to use all your gold training huskarls. Attack red, kill the gates and kill the army. Destroy the town centre.

Train up more huskarls, sell your wood. Buy enough food to buy all huskarls with the remaining gold. Purple will attack soon, on one playthrough they actually destroyed my initial base which is very annoying.

Park your dudes in the exit of red, send 5 huskarls to attack the nearest yellow gate. Lure his scorpions and skirms out of their town. Kill the scorps and run through the gate when their skirms open it for you. Make a dash for the upper gate, destroy it and destroy the town centre.

Train more huskarls. Sell extra wood and make more huskarls. Keep 8 behind in case purple tries to attack yellow or red. Send 5 huskarls to attack the gate of blue. Wait for their army to come (especially the onager). Rush through the bottom gate when a crossbow opens it for you and kill their army. Kill their onager before it can kill your dudes. There is a monk worth killing. Once both are killed, kill the second gate and kill the TC.

Train up more Huskarls. Sell wood, buy food. Train more huskarls. Free the gold markets with your huskarl ball.

Train more huskarls, sell wood, buy food. Move on cyan. Cyan has a smaller army but has a castle. Wait for their army to appear at the front gate. Kill it and run past the castle to the second gate on the right. Destroy the gate and kill the Town Centre.

Sell wood, buy food. Train huskarls. You should have good income now. Send your huskarl ball to clear the mines. Lure them away from the towers for minimum casualties.

Sell wood, buy food. Train huskarls. Spam 60, send them to the gate of purple. Start attacking. When Purples army shows up, move inside and get into the second gate when some purple unit lets you in. Kill the town centre plus towers and move south to find the rest of the villagers. This will stop purple producing many units. Then move north and take out the production buildings.

Meanwhile, sell wood, buy food and train huskarls. Next 60 unit ball, send to kill the gates near the castle. After the gates go for the castle.

Purple might keep attacking your old towns. I find that 8 huskarls should be enough to kill these groups of rams. Good luck with the achievement. It's essential that you only train infantry so good luck!

Age of Empires II: Definitive Edition (PC) - Achievement - Numerous as the Grains of Sand

Win the second Alaric mission "Raising Hellas" while training no military units aside from infantry.

There's a reoccurring theme in this mission - Attack and take a town and it will generate resources for you. These towns have gates / walls, so the idea of only being able to train infantry for the achievement can be somewhat of a hindrance, but it's still not all that difficult as Huskarls (which you will have access to) are pretty effective against buildings, so can use them rather than siege weapons.
You start with a small settlement with a few military buildings and a blacksmith - Seen as we'll only be using infantry, you should upgrade the things only relevant to them. You can't train Huskarls until you take the first town, so feel free to train a few pikemen / swordsmen to go along with you at the start. There's a Roman group in between your starting location and the first city, but you'll be able to take it with your overwhelming numbers, despite their cavalry being statistically superior.
All you need to do to 'sack' a town is destroy it's Town Centre, so you don't need to waste time destroying buildings or even towers for that matter. Just destroy the gates when necessary and head for the Town Centre, killing any troops along the way. One tactic I use that may prove helpful is if enemy troops come to a gate and open it, rush all your troops in so you don't even need to spend time destroying gates. That being said, it's still very much achievable to complete this if you don't do this. There's even a couple of side missions along the way which you can complete whilst still doing the achievement. Just keep pumping Huskarls in to your army when you get resources from the towns you've sacked.
The only notable threat is that enemy troops (including potentially more than one Battering Ram) come from behind and will begin destroying buildings in the towns you've previously sacked. They move fairly slow, but keep an eye on them as you don't want them to leave you without the ability to produce units.
Once you've sacked all the towns, you'll be told you then need to defeat the Romans. Just attack their base using the same tactic you've been using and make sure to destroy their Castle - You'll need to do this to defeat them, but you overwhelming ball of Huskarls is very capable of doing this with relative ease.