One of the fastest and easiest ways to level crafters is by doing levequests. There are even some leveling sections where you can just buy the leve items off the market board and the gil you're rewarded tends to be roughly the amount you were spending (mythril ingots come to mind, for this).

If you're out of leves, just grind out crafts like you would any other job. Since you wanted to level gatherers too, you could knock out two birds with one stone.

Other options for exp are your grand company supply/povisioning turn-ins, but those are only doable once a day.

Re: Ixal quests
You get an infinite number of tries, right? If you fail to HQ, you can just pick up another set of ingredients, if I recall correctly. Just keep trying until you succeed. Other suggestions are going to be your usual, use your best crafter, get better gear, try different cross-class actions, etc.