You get more orbs from killing yellow bar enemies, so its a good idea to try to bomb several yellow enemies at once. A recommended way to do this is to start the earth based strike, The devils Lair, but from the start go back through to the steppes area (patrol spawn) and find the three yellow bar hive enemies that are in an underground room. Killing these three at the same time with a nova bomb will get you 7 orbs. Repeat it and then go finish the strike.