I tried the Sand Worm and got wrecked. Eventually he just started chaining Quake and I was dead pretty quickly.
I still think the Sand Worm is the easiest of the 4. This would be my generic strategy:
Take Wall as your Roaming Warrior. Sentinel Grimoire or Stoneskin II (Tyro or Y'shtola). Cast the first right away to buy yourself time to get the rest of your mitigation up. Recast Wall when he hits 50% to get yourself to the end. Load yourself up with Ice and Water attacks. This is the only boss of the 4 that has an easy vulnerability to exploit (The Crane from VII is weak to Lightning but only after the arms are gone). So you can take chunks of his health away with these attacks. If your hones are low than hold onto to these attacks until he's around 50% dead and then unload on him. Magic Break(down) is really helpful to take the sting out of Quake. From my experience he doesn't cast it early but seems to start chain casting it when he gets low on health (data mining shows otherwise but this was my experience). So that's when you need to get the Break on him. Shellga doesn't hurt if you can fit it in.
A typical load out for me looks like this:
Melee 1: Physical ability of some sort (think Watera Strike or Blizzara Strike) and Mental Break
WHM: Curaga and Diaga
Support: Power and Magic Breakdown
BLM: Two high honed spells that can hit a Vulnerability if possible
Melee 2: Random ability and Armor Break
For Melee 2 consider Paladin Cecil with Shellga. If you don't have decently honed Breakdowns yet than that Support person can be anyone with regular Power and Magic Break.
As for his attacks: His Swallow Attack is annoying but it's not fatal. The Slow it leaves behind is the most annoying part of it. His headbutt is physical only and is easily healed through. That leaves Quake. According to data-mining he uses Quake constantly throughout the fight at 40% likelihood per round increasing to 50% if someone is Swallowed. If someone gets Swallowed you need to break them out as quickly as possible. You need to do 10% of his remaining health in order to break someone out. So someone swallowed early on is much harder to break out than someone swallowed later in the fight. So hold back on your Hones (Water and Ice) if you don't have a lot of uses until someone gets Swallowed and then go all out to break them free. Once he gets down to 50% than try and burst him down.
Last piece of advice on the fight. If he gets TOO Quake happy than just S/L and try again. You should eventually get an attempt in where he doesn't Quake so often and you can get the win that way.
You get the MC2 just for completion so if you can't Master it than don't worry about it. Getting the MC2 is more important than getting the Upgrade material and Mythril. It sucks leaving any Mythril on the table but the MC2 is your goal. For someone with zero MC2 Crystals than my advice is to spend the first on Cloud since they love him, VII is a very common realm and he's just all around strong. If you can get a second than my advice would be Rinoa since she'll be the only level 80 BLM for awhile. I'd only consider Rinoa if you can give her a decent weapon through (like Valkyrie from the beginner picks since she can't use Storm Staff).
