You will get this over time or you can grind it out in under 2 minutes on any mission. Before you start a wave, walk over to a ground trap and click
to open the menu and build a trap with
and sell it with
. Repeat 249 more times to get this achievement. If you do this in before a wave then nobody will attack you plus it sells for the same amount it cost to build it as long as you dont upgrade it.
I went to an empty trap and spammed
,
,
,
,
,
and repeat in that order 248 more times