you should kill as little as possible with the sniper so that your gunmen get all the xp. I dont really think its that difficult. i made it to night 17 without much effort
no for the last level you get the upgrade and then build two drones and have them bomb the pit and then evntually get four of them and a sam and then you can just sit rlax and build up your energy
 "Dead of Night"){kind=icon}
 "Bloons TD 5"){kind=icon}
 "Elephant Quest"){kind=icon}
 "Creeper World Training Sim"){kind=icon}