Aim (most of the time) for the death stars middle. That will take care of the little ones. It will start slowly descending near it's death on you, just hang around the middle so it doesn't trap you and once it's on the ground it wont move any more. That will give you enough space for the final shots to fend off the baddies and kill it off.
At around level 23, the numbers get too big and fly too fast to shoot them down in time. Even with 3 spamming. Give us multiplier cannons (like 7x & 12x) so we can shoot down big ones
Needs formation saves!
The winning strategy is to start out with a few high level units, (use the upgrade button) and just build your army in bits and pieces with more and more high levels. When you add a new guy, make him at least a level 4.