I think maybe you just had the wrong approach- Especially when a quest says you should try it with a light party.
I duo'd the level 30 job quests for DRG, BLM and BRD with a 36 LNC + 34 THM and I helped a friend do the WAR one with my 36 LNC and her 31 MRD...

It's called prepping up for a fight. I bought potions, food and unguents.. And for the "having another job at level 15" requirement, it's only normal... You don't want to be stuck on your new MNK and not have any cross-skills to equip, right ?