I got this while playing with the following settings:
Map: Flight Plan
Mode: Poison Core
Focus on on gun towers near the top/middle, and upgrade them when possible. Put a missile tower in the middle, and upgrade that to at least level 2. A temporal tower in the middle (again, upgrade when possible), and one near the exit.
The last level has two crashers, wipe out everything but those and let one pick up the core. Sell all towers (except the temporal tower near the exit) so the crasher with the core does not get killed. When it is just entering the cave (exit), use the orbital laser.