Do you really think gunning down some 20 year old Guatemalan girl clutching her infant daughter is going to get any public support?
You don't need to resort to anything near that.
Simply task ICE to enforce our workplace laws. Once the criminals who are hiring them get hit with fines of $100,000 or more, they'll realize that employing illegals isn't as good a business practice as they once thought.
When the jobs dry up, so does the incentive to stay and the vast majority will deport themselves. Use the revenue generated from the civil penalties to build a double fence along the border and patrol that with existing border patrol.
NOW, once you've heavily de-incentivised the idea of crossing the border illegally to find work, you can operate under the assumption that anyone sneaking across presents a serious threat. You don't need gun turrets, just patrol a defined area with UAVs and blast anything in suspicious.
Why stop with the employers of illegals? Consider all who benefit from illegals? For instance:
Schools that fill their roles to get more govmint money
Latin American Countries who receive influx of money from illegals here in the U.S.
Illegals themselves
Some churches
Others?
The idea is to turn their assets into liabilities as you suggest. Make them feel the pain. End result is self deportation.
Sounds like a plan - makes sense, workable. Good luck getting that to fly! Although maybe there is a spark of hope...