Chain as best you can on this level. For the first part with all the small white enemies, stay in one position and slowly, in very small increments move backwards. The enemies will swarm toward you, but end up right in front of you. (Check the youtube video)
For the rest, just try your best to chain as well as you can. I had trouble doing this on easy, so I had to bump it up to Normal. The fire that comes out of downed enemies will help fill your charge and also give you extra points.