They Are Billions

This one isn't all that hard, despite sounding like a nightmare!

Ballista is your only weapon until the final couple of waves come, you don't need to upgrade to the executor, or start laying down shock towers until the last few waves, you'll get a feel for when the waves start getting too much for your basic ballistas.

Map choice
Forzen wastelands is the obvious choice for this achievement
- the scarcity of farmland is a non-issue as we will not be building an army, so we only need food to power population growth
- the movement speed penalty is a non-factor here, and the reduced activity from the infected that balances this "penalty" really helps in the early-mid game phase
- the natural choke points will minimise the area that you have to defend
- the map will more often than not be laid out as a series of lanes rather than a vast open plain, which is an ideal fit for ballista-based expansion

Expansion
- Lay a ballista
- Expand by placing another ballista 5 grid places away from it, the first ballista will cover the new one while it is being built. Expand aggressively in multiple directions at the same time.
- If there are lots of hostiles close by, spend a few extra gold and wood to build a wooden barrier in front of the new ballista
- Destroy/reclaim ballistas once they are three deep (ie only keep 2 layers of ballista)
- As you are not building an army food will hardly matter, and until you start upgrading your ballistas to executors in the mid-late game upgrade phase you will hardly struggle for energy either. Money will be your limiting factor in your expansion here, so every spare penny you have should be going straight down into new houses. As the time of a new wave approaches start to keep a 1000 gold kitty in reserve so that you can add a couple of new ballistas as soon as you know where they will arrive on site.

Waves / Swarms
- When waves come, wait until you see where the wave spawns, and lay an extra ballista (or two), and some walls as a safety net. Grab your rangers and bring them to your border. Use 1 ranger to kite the spawn within the range of the ballistas.
- If you're not sure how to do this (kite the swarm), here's the basics:
- Zoom in!
- Take your ranger and run him parallal to the incoming swarm, you should see the swarm make an about face and start to follow your ranger
- once you've picked up the swarm, loop back round and zig zag your way back to your defensive line, picking up as many stragglers as you can and bunching up the swarm
- run loops in front of your ballistas (don't let your ranger stop to take pot shots, just keep moving and keep the swarm following you -- they don't lose agro and will ignore the towers) while the ballistas nuke the swarm
- If you don't have mouse and keyboard this (kiting the swarm) will be almost impossible to do imo, in which case you will want to instead lay down multiple layers of walls and use spike traps, but without the ability to kite the early to mid-game swarms around what will be fairly limited defenses at that point I think you're going to struggle.

Endgame prep
- two executors positioned alongside a single shock tower will handle most endgame waves. Build your walls 2 (or 3 on a diagonal) deep, and add stone towers every other tile to make your walls +1 deep.
- start laying wooden spike traps (and barded wire traps if you are sitting on max iron only), as much as damaging the incoming waves, they buy you some time to react and re-inforce your defensive positions.
- keep resources in reserve, once the waves start making their beeline for your defenses build a second layer of defenses behind your first line of defense at the impact points. If your walls are 3 deep, and you built your first line of defense flush to the walls, you can build three layers of executors that will be able to fire on the enemies as they hit your walls. Nothing will survive that.