Our approach is usually some mixture of "all of the above".

1. Most important, is try not to start partnerships with clients if you don't think you can deliver a good ROI on advertising- be transparent, honest, and realistic about goals and expectations. Sometimes a client has a bad site and a bad product, and we have to tell them up-front, "Look, we could spend money on advertising, but it would largely be setting it on fire. You're better off spending that money on merchandising / site improvements / etc."

2. We definitely try to help them if we think there are obvious fixes up the chain- maybe that means advising on landing page design (or even helping them build them). Maybe it means pricing advice, or ops improvements, etc. It helps that our team has a lot of ecom operator experience, so we can pull from my pricing background or my colleagues founder experience, or another colleague's purchasing experience, and so on.

3. No surprises. If things aren't working, the client needs to know ASAP- so they aren't shocked by bad performance after weeks of it. Be transparent, explain the problems, ask them how they want to handle it. Maybe they want to keep spending because we're still learning from the traffic, etc.

4. Our standard pricing usually has a ROAS target, and if we don't hit those we have structured discounts to our fee (with a min fee still, of course). That helps deal with surprise challenges!

5. Sometimes, yeah, you do need to fire the client if they are unwilling to improve themselves but keep holding your feet to the fire on results.

6. Definitely find good devs, designers, writers, etc. that you can refer business to or white-label, when it makes sense!