Still trying for 5.
I found the 2nd barrier not too difficult though. Place two sentries between the two electric walls facing away from the hive but have one facing more to the right to cover scorpions that jump up onto the truck. Now just stay around the hive area and stay alert.
Majority of aliens will go for the sentries then get electrocuted but for hunters that target you, run through the gap between the walls and, again, they will be fried. Pull your sentries back if a meteor lands in front on them else they'll most probably break.
This method seems to work for me as the electric does most of the killing and keeps bullet usage to a minimum. But if you are going for an uber high score this may not work as well due to you taking electric damage yourself.