When your last weapon breaks on day 3 and the final enemy is at 1 hp and you cannonball it, you still get full EXP, but you do not have to go through the time loss that is opening the chest or your soldiers walking to the chest.

This can save 10 seconds on day 3 and about 8 seconds on day 4 if you use the cannonball on the 3rd enemy on day 4. So, with 5 seconds of better than world record gameplay and 2 cannonball glitches, sub 10 is possible

That's actually pretty smart, yeah, nice find.