The Lost City - Burn Baby Burn
Light 5 signal fires.
This is an easy challenge to complete, but you can only light the fires with a trebuchet. When you're first sent to commandeer one (when trying to exit the town area), you can rotate your field of view to aim at four of the five pyres.
You can aim at the final fire with the second trebuchet. The fire is to the right of gate you're trying to shoot down.