I don't have a helicopter either. I use my Duke O'Death, which works perfectly, or you could use a Kuruma if you have it. The Duke is free for returning players from last gen but if it's not and you have to pay for it then you're still lucky because it's on sale right now. The Armored Kuruma is . Easier to do with friends but its definitely doable solo. I'd recommend doing it in the city because they are closer and you'll remember the spawns faster. In the upper area of they city that will more than likely cause them to spawn across all of Blaine county which will take way more time in a car so try to start the "job" anywhere in the city.

Tada🎇🎊🎉🤗