DON'T READ THIS IF YOU DON'T WANNA KNOW HOW TO DO THE LAST BIT!!
Once you've fired all 5 objects from the cannon, try to fire it again, and the little guy will climb inside. Click on the yellow arrow, and another one will come along. Point the cannon upwards and fire it.
Just as the little guy approaches the spike, click it. The spike will dip down momentarily and he won't get speared.
Once you've done that, click the lil chap and he'll push the bell thing across. Click three times on the bell and it will fall off onto the end of the spikey see-saw thing, launching the rock into the air (the one you fired at the bell early on). It'll fall onto the landmine and detonate it.
Once the stickman has gone into the building, you need to open the window. He'll slam it shut and that'll cause some debris to fall below. Time it right, and it will land on the dog/saw thing (whatever the hell it is) - try opening the window just after the dog/saw has hit the ground.