Bombing campaign is a special combo attack made using princess and the
cannoneer. It is very useful in various parts of the game which is why I recommend everyone to have these 2
mercs at least in the
merc room if not in the main party.
Separately princess heals all ranged
mercs, gives them defense and has skills to block or reduce damage of enemy ranged
mercs.
The
cannoneer is a cannon
merc who can cast smoke bomb. This is a skill which reduces accuracy of all enemy
mercs so quite often when they attack your
mercs will just dodge.
On top of the above, these 2
mercs combine to produce the devastating bombing campaign. You can see an example and further explanation of it on the below picture story I have prepared for you: