Poison Core red medals

[ReadyMan]

Any tips on how to get the red medal on the poison core game mode? The blasted power core keeps destroying the aliens behind it so that I dont get any points/resources for them.
I've had trouble with this mode in all the levels I've tried so far, but the "surprise party" last level is particularly hard.
Thanks for any help!

[datacz]

Did you got "Untouchable" achievement yet?

Simply do not let the core to be taken. How? Make longest path possible, use right towers at right places. Use Command Towers and concentrate firepower around them. Green towers shape the path red are killing.