(and other tips)
You can use your engineers to draw away some viruses from an undesirable location. This can be either used to clear out an area (although this can be tricky), or to bolster your troops. Basically, you move an engineer near some viruses, when you see nearby viruses move towards it, control your engineer to move away to a spot where you have laser-toting Darwinians. The Darwinians will kill the viruses following your engineer, and the engineer can then collect the souls. The trick is to not pull too many, or you risk getting your engineer destroyed. You can do this to pull the viruses from virtually any spot on the map. They'll follow through water, over hills, anywhere. Once they've locked on, you can take them wherever you want, for whatever purpose. Rather than order your Darwinians into a mob of viruses away from your incubators (and engineers), have the mob come to you on terrain you control.
I'll add some more tips later. In the meantime, feel free to post your own!