If it seems hard to aim and you die early, invest in smart missiles first. They provide a steady income of ether. Afterwards, whatever floats your boat.
all bosses are able to be killed. for those that are hard, make sure to wait till boss is not firing and then use the time stop ability and fire on it.
DMD, use shatterbolt in the beginning and use electrobolt, when he has 100 focus use block or subversion on yourself. Keep on doing that. Only for guardians. It helps if you 'eat' your items.