Is there anyway to get more energy?

Just got the game and I am stuck grinding on this level. I can't get passed some of the cobras that strike multiple units.

I try my best to play apps as f2p but will spend a few bucks if Im really enjoying it or fixated on something in it. Im trying to get s9me mote energy to keep trying this level but dont even see a package or deal for any extra. I know there is the few ads and gems you can spend for a couple extra energy but that is it.

Is there anything I have not unlocked or something?