If you wait for them to collide at one spot, the maximum kill you get is 15. However the mission/quest part 6 requires you to kill 10 of the minions in one shot. Here's how you do it..
1. Find yourself a spot; try not to zoom in too much cos you don't wanna re-adjust your scope after that. Preferably the spot doesn't have much clickable objects like stationary characters or buildings. I recommend a good cross junctions where the roads meet. Center the junction to the middle of your screen, which will help later..
2. Camp for Stewies that come in your direction (TOWARDS the middle of cross junction or screen)
3. Tap them so that they stay in the spot squirting ink. You wanna keep as many there as possible.
4. When there's sufficient Stewies stationary in view, quickly tap the bomb button so that they don't run too far from your AOE (area of effect).
5. There you have 10-15 Stewies bombed in one shot! GLHF
What maze? I don't even need to destroy my landscape to kill 15 Stewies