Well , a friend of mine told me to do this :
on the last wave kill all the aliens less the devourer , let the devourer go to the corner , and then you click him attack without pet and donīt move your ship or change conf. or change ammo , you must do nothing and wait for the devourer get killed , then you stay without moving and you will get the havok. I tested it 3 times and everytime get havok.Test it and comment.Sorry for my bad english iīm argentinian.