First, you need the Hunter's Monograph as others suggested. To make this available in the bazaar, if you don't have it yet, go speak with Gatsly in the Muthru Bazaar. The monograph is 18000 gil.

On my first playthrough, It took me until a 670 chain chaining both Tartaruses and Cerberuses in the Walk of Stolen Truths in the Feywood before I got all of the Hell-Gates Flames I needed. This was with the monograph. I read that chaining just Cerebuses is better although doing the Tartaruses as well helped to get the chain level higher. A Tournesol guide (one posted on a forum) I used had advised to chain Tartaruses as well, but people had reported results with chaining just Cerberuses.

As lte mentioned, increasing the chain level doesn't increase the drop-rate for "monograph drops" as they drop at a fixed rate.

One thing to try would be to chain just Cerberuses and don't chain more than around 30 of them. Touch the save crystal at Antiquity's End, and then zone south below that to Redolent Glade. After that, go back and chain around 30 again.

I hope this helps.