TIP #1
Go as fast as you can at the very beginning and stun the mothership... or else you will lose the first survivor... if you can't get there in time.. you can try and risk saving them from the volcano... not fun.
TIP #2
Any time there is an orange enemy... kill it! It will not respawn again.
TIP #3
Take risks.. there is no bonus for extra lives remaining.. as far as I know!