Close Call

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Save a core that is less than 10 seconds away from the map's exit point.

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This may occur naturally while playing the Story mode. If not, it can be easily obtained on Chapter 1 in Practice mode on any difficulty level. Start the game and let the first wave of aliens reach the first turn. Build a single gun tower on the outside of the path, as close to the exit as possible, and upgrade it to a level 3. The initial wave of walkers should be back in range right about the same time the second wave of swarmers are entering the map. Your single gun turret should kill the walkers as they come into range, which is about 10-15 seconds from the exit. The swarmers should pick up some of the dropped orbs and since they are much faster, they will already be within ten seconds of the exit. Your gun will kill them also, and the achievement will unlock at the end of the game, win or lose.