Found an interesting exploit; Upgrading a unit while 2x attack is active will give that unit a permanent 2x attack state, it doesn't stack with another 2x of course, but the real kicker is that the bonus spreads to other units of the same upgraded type. It might've been because I did it to my first unit though, or at-least it did this to my first galaxy turret by coincidence.
*after losing to sudden zombie horde cause I held off on getting the submachine gun*
It looks like this game,
*puts on sunglasses*
Is over.
YYEEEEEEAAAAAAAHH