During the first few levels after your forms are taken there are a number of spawn nodes that resease basic drone and gunners. These pose very little threat. When you find these you shouldn't destroy the node. you can continue to kill the drones with out being damaged. If you only use X you can get more than 150 hits in a chain before the node runs out of people and destrys itself. In the beginning of the game this is the best way to get alot of plasm early on.