The goal of the game is to finish all three challenges without losing, but the order of these levels is random. When you complete a challenge, it wouldn't be selected again until you start a new game.
For the robots levels, they are not spawning indefinitely. There is a fixed number of each type. Maybe the numbers should be displayed to not cause confusion. If you want some strategy, I would say kill the bigger ones first while doing a maneuver to avoid smaller ones.
Bullet hell seems to be fair because there is not a strategy I can think of if it was harder.
For the targets game, you have to try staying in the middle then move when a taxi spawns to intercept it first before firing at it.
And yes, the game is hard for casual players but it does not require hardcore players to win it.
Anyway, thanks for clarifying these points and I hope you enjoyed this experience!