Main mission won't progress...

The timer doesn't start until you touch the screen, so I find the target's location and an area where small things are clustered near the start. Useful items are scattered near the start for consecutive clears, so grab the bottle-like thing that can enlarge holes first, then take the 2x EXP item, and if you drop a lot of the small things, you can make it bigger more easily. Other than that, if you make sure the target doesn't go to a corner and you don't drop bombs out of momentum, I think it'll generally work out.