Dont farm Long Guis. If you have trouble with Adamantoises you're not close to Long Guis. And yeah, summoning takes out an Adamantoises legs immediately, extremely helpful.
Once you have 3 ravagers stagger him to 999%, consider using Sazh's blitz, with his Hyades (tier 2, MASSIVE attack). It hits for 50-60k per hit, and at that range against such a huge opponent you get a huge amount of hits in.
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Current playing: No More Heroes