Well "best build" is your opion really i have friends who think warriors are the best,ect...
but clerics are great at soloing and always wanted in a party
and of course you can have nukes with light armour....you just dont get the 20% moving speed and 20% less mp used when casting them that you get when using robes
with light armour you get 10% less mp used and 10% moving speed(but less mag def and more phy def)
personly i perfer light armour
and ya they have lower dmg than wizard and warlock(well at least after warlock debuffs) and not sure if it has higher dmg than bard
but they also have one of the strongest(or the strongest not too sure) nuke in the game,offering and pure offering(upgraded version)
and yes im a cleric
