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.