Guncraft Blocked and Loaded

This is best done in conjunction with the 10,000 kills. Start by creating a custom Prefab in the foundry, using the $1 material, making it as tall as possible, and keep adding on blocks until it's 2000 blocks in total. Save that prefab 10 times under 10 separate names (Build, Build 2, Build 3, etc). Once they're all saved, go into the Edit Classes menu and select "Prefabs". Take out all of the ones you had already selected and highlight the 10 with 2000 blocks you had previously made. Make sure you save and apply these changes, and that you make your character an "Architect" type, with a manned and AI turret killstreak.

Make a Deathmatch game and bring in the boosting buddies you'll be killing, and make sure Infinite Resources is OFF. Now, while you're allowing your turret to rack up kills and money for you, you can go along the edges of the map building your prefab. The cooldown is capped at 2 minutes, and each build is only about $400. When you run out of room to build, get enough kills to end the game and repeat. Should only take about 3 games if done correctly.

I believe have found the most efficient method to unlock this achievement:

How efficient this method will be depends on how far into the game you already are as mechanics like the prefab editor as well as different game modes will only unlock at certain levels. Best to grind some XP before attempting this.

IMPORTANT NOTE - Having the "Infinite Resources" box checked will void the placing blocks achievement (although oddly enough this seems to be the only achievement that setting effects)

First you will need to create a 2000 block prefab (that's the block limit) in the prefab editor then save it 10 times under different names - You can make this go faster by using the D-Pad to change the width, length and height of the blocks you place, to reset your brush to 1x, hold in the D-pad button for that direction. Also make sure to use the cheapest material which will be on the far left of the block selector.

It's best if your design is as wide as possible rather then tall as prefabs build from the ground up.

By hitting you can see how many blocks are currently in your prefab when it hits exactly 2000 you are good to go. Save it under any name then save it again with [name] 2, [name] 3 etc.

You can equip these prefabs in "Edit character and Classes" > Select Prefabs. Uncheck all the default builds then check all 10 of your prefabs you just saved.

Now when you load into a match you need to earn $5000 in order to build each of your $500 prefabs.
2000 blocks X 10 = 20,000 blocks placed per match (as long as you don't accidently build the prefab on top of each other).

The BEST way to do this would be to create a custom map with respawning Steel blocks (press twice in the editor for respawning blocks) for money generation and place all objectives so it can be played on every mode (open the block selector using ) after you are finished save it. Alternatively you could use a non custom map however those are far less efficient.

Start a Capture The Flag match on your custom map with the score to win set to 1.

Select the assault class as they have the miner perk which will double the $ gain from every block destroyed.

Go into build mode and destroy the steel blocks until you have $5000 you should get $20 per block destroyed.

Once you have $5000 place all 10 prefabs ensuring they don't overlap and wait for them all to build.

Once they are all complete capture the flag ending the match and thus bypassing the 10 minute cooldowns of the prefabs.

Repeat matches on this setting until you have the achievement.

[Will create video guide soon stay posted]