The quickest way to attain this achievement is to use the dynamite with which every attacking team member spawns. This can be done on any map.
Start a private Stronghold match with no fewer than 4 people. Set a meeting point, have the defenders group together... and BOOM! goes the dynamite. Repeat for each attacking team member.
Note:
If you have 4 or 5 people then have three posse-up. With 6 people everybody will get it in the same game. If, for whatever reason, you start to run short on time, have the attacking team capture a position.
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Any questions, comments or any other feedback please leave a comment below and I will get back to you ASAP! Good luck!