Attacking Fortifications is the easiest EXP ever.

 

After getting the Bronze Star, I went and did another Fort Karugo-Nargo Campaign - the result is in the screenshot - my highest EXP gained yet in campaign.  And to top it off, it was mostly from attacking Fortifications on my WHM/NIN.  Mobs eventually came in so I did get hit a few times (and raised a few people), which helped the exp go up I'd imagine.  I got a merit point from it too. 

Gogo melee WHM :P