You need to understand the order of destruction of targets.
Time stops when the shells run out or all the soldiers are destroyed.
You need to destroy valuable items before you destroy the last soldier.
For three barrels you need to destroy all soldiers and all valuable items.
To open the lock on the bonus level, you need fuel. If there is not enough fuel, you can improve the result on the levels or buy barrels for the game currency.
A lot of combinations. There are more targets than projectile. You need to use objects and mechanisms. Goals are moving. Physics. All items are destroyed. Objects interact with each other.
Everything is fair in the game. Use hints.They will show you how to complete the level.