Here's a short solution for this achievement:
Max Level for the fabricator is 4 and to upgrade it, you need to collect the energy that the enemies drop when you kill them AND use it afterwards to build barricades or other reinforcements from the fabricator.
Everytime your fabricator does an update, you will be able to upgrade your previously built low-level sieges by picking them up with
and upgrade it afterwards with
. This will add the amount of energy used to the level-counter of the fabricator.
Repairing broken reinforcements or reloading tourrets won't count towards fabricator updates.
About the Levels:
Level 1 ==> 2: 30'000 Energy
Level 2 ==> 3: 90'000 Energy
Level 3 ==> 4: 120'000 Energy
How to do it (Easy way):
You need: 2 players at least (Just to keep the difficulty down)
Classes: Engineer + Scout (Rest may vary)
Difficulty: Easy (Lowest)
Map: The one you like the most / you are best in
Startup the game and work yourself through the waves. It's necessary that the scout is collecting all the energy (His class gives him a collection Bonus on dropped enemies). Make sure that you let the last enemy live. That's important, because the bonus is lost, when a wave is finished. Collected energy in the pause between the waves is the same for every player
. If you need some bonus energy, go for executions. An execution gives you a small bonus on your energy-counter. It's up to your playing style, but we prefered to not buy anything from the fabricator until we had finished wave 20. We got around 150k energy at the start of wave 21.
To go fast towards the level ups, invest in tourrets (self firing) and add some traps and barricades to enhance their effectivity. Anyways, we hit the Level 4 mark around waves 31-33.
Engineer lvl 1: Bonus: 100% Effectivity
Scout lvl 2: Bonus: +40% Shotgun Damage
Sniper lvl 2: Bonus +60% Ammo for Snipers
You see, we were pretty low leveled but with a familiar map and good communication it's cake.
Bonus Info: We had the Fabricator on level 4 and i invited another friend to play with us and he got the achievement mid-joining.