Thanks to BUTER01 I realized that I was killing the bugs at the start too fast which was spawning more bugs. You need to kill all lanes at the same time. The killing of the bugs without your troops is actually quite optimal with a little bit of luck, except for the upper lane where there is some bugs remaining. So I block one of the troops line of sight to simulate a battle without one of the troops and at 0:50 I move out of the way to kill remaining bugs faster.

The grenade during the second part could have been much better.

Don't look at the SOB, there are fake golds in there.