How do I open a chest?
Bolts are unlocked when you succeed level I3
Bolts is gem ehhancement.
With bolt, a gem ignores armor when the gem is located in a trap. The enhancement don't deplete when the gem is in a trap.
When the gem is located in a tower, you got a certain number of powershots.
To answer your question, the chest will open when you have a gem enhanced in a tower with bolts that shots it 3 times.
More details here