Best way to farm coins?

I’ve been playing for a few weeks and I’m a dolphin. I’ve spent some money and got some packs so I’ve been able to push to have access to tier 11 (get destroyed pretty quickly) and in tier 1 I can make it to about round 2100. I want to open up more upgrade features but they cost 500M or 1B coins! Do I have to keep doing 10-15M runs to stack up or are there any actually strategies to help farm coins a lot faster than going to round 1000 on tier 2? TIA