According to Beseiged

BLM:73 WHM:53 SMN:36 THF:70 WAR:56 RNG:36

Doing Besieged is like the most easy and exciting exp there is. You find yourself a party and basically you start to heal. That's right. Don't think that your BLM or RDM has any chance of landing more than 100 pts of damage on the mob unless you cast like a Flare MB or something like that. These monsters have huge HP too so getting them to fall over is a monumental effort.

The best thing about Besieged is that it doesn't last too long and that there is an anticipation effect beforehand too that really adds to the fun.

Besieged was a great idea.