Personally speaking, I found that in this game, if you only use the normal zombies with no special zombies, you could save the energies by your special zombies. In this case, it is very important for us to farm as much as possible. After 3.3 released, I only farm brains in SF hard. Because it only contains the snipers in this map. You could use only your normal zombies in this map (hard). For the borrowed unit, in my opinions, if you could borrow some infectors or Reanimators or farmers or rachers in your guild, it will be very much helpful for you to farm in this map. You could do this without any thinking. Thank you very much. If you have some tips for me, please reply as followed. Thank you very much.