im assuming this must be one of the bonus levels, Allies Enclave with peter and susan. i have never beaten this level. what i try to do is have susan kill a few wolves and get a heart to restore her health. i've gotten pretty close to beating it, so it works if done right.
also, play the main game and go thru all the levels and buy all abilities. this will help immensly. i love Shield Slam! also the ones that susan can shoot faster are helpful.
hope these ideas help!