I'm fairly new to this game, just started toying with it a week ago.
I've numbered the cannons by build order.
Cannons 1 and 2 were built immediately and upgraded to impact cannons as soon as possible. Then cannons 3 and 4 were built as money allowed and upgraded to quick cannon IIs. I focused on getting 1 and 2 to ice 2 as quickly as I could, then getting 3 and 4 up to QC4. After that I built 5 and 6 and ran them up to QC 4.
From there, every tower was built and upgraded all the way, one at a time.
As for targetting strategy, I generally only targeted healthy ants inside my kill zone. This focused my fire on ants that I could keep pinned on the nest longer, and allowed those who had broken the line to wander back into the kill zone because my cannons were busy with other targets. My goal was to push all the ants to as low health as I could, then kill them one at a time. I finally lost when my luck ran out and five ants died within a few seconds of each other, spawning five new ants with insane amounts of health. I simply couldn't do enough damage to stop them, and the cake carrier came back at full health.