Flan are Black Mages, and happily cast high level ~aga spells and Ancient Magic.

Flan have a high resistance to Physical attacks, but are weak to magic, and take much more damage from spells then most monsters. The Flans seem to have modes where they change shape, and get 'spikey' during which they get even more physically resistant, and weaker to magic.

Synergism will wake any Flans that have been slept that are drained from.

Flan aggro to the use of Job abilities from a fair distance.

This spell can be easily soloed by a level 95 BLU. 

There are a number of Flans in Abyssea - Konschtat just norht of the #06 port in the G-4 square. The fights here are very simple, pull the mob, and feed it TP until you see the spell. It can be helpful to have a silence set to keep the flan from casting, and a dispel in case it gets blaze spikes up which is annoying. So see the spell and learn. With buffs these fights are 100% safe solos.

Once you zone into Mount Zhayolm from the staging point, head to a Havlung Zone line at K-8, marked with a '2', and a red path on the map above. You'll need invisible to get past the clusters, and if it's night, sneak to pass the Ghosts. There are Crawlers which will aggro to sound, as will the Clots that are in the tunnels. Take proper precautions.

Once you zone into Halvung move to your camp at H-9, you will need to pass several Crawlers to reach it. So use sneak to get past them.

Camp is right before a gate that you can click on to open. Just past it Three Black Puddings and Three Trolls wander. Wait until you can make a safe pull of a Flan without any Trolls seeing you and grab your target. If you have a ranged wepaon you can pull through the gate. Flan do not link. The Flan may choose to open with a spell. Simply run out of range, and wait for it to come to you. Bring the Pudding up from the gate a bit as to put some distance betwen it and the others in the pull zone. Do not go too far, as the Crawlers are North of you. The Pulls are the hard part of the fight in this camp.

The fights are very simple. The Blue Mage should be ready to Stun at all times, to stop the casting of the Pudding. They are annoying to silence, so stick with the stuns. 

If you accidently link a troll the best option is to sleep it. Apply sneak, and zone.

Proceed to the Halvung Zone at C-9 in Wajaom Woodlands. Sneak and Invisible are needed. Or Grab a Chocobo in town and ride out safely.

Once you zone into Halvung move to your camp at H-11, you will need to pass several Crawlers and Bats to reach it. So use sneak to get past them.

Camp is right before a gate that you can click on to open. Just past it several Black Puddings and Trolls wander. Wait until you can make a safe pull of a Flan without any Trolls seeing you and grab your target. If you have a ranged wepaon you can pull through the gate. Flan do not link. The Flan may choose to open with a spell. Simply run out of range, and wait for it to come to you. Bring the Pudding up from the gate a bit as to put some distance betwen it and the others in the pull zone. Do not go too far, as the Crawlers are about and will aggro. The Pulls are the hard part of the fight in this camp.

The fights are very simple. The Blue Mage should be ready to Stun at all times, to stop the casting of the Pudding. They are annoying to silence, so stick with the stuns.

If you accidently link a troll the best option is to sleep it. Apply sneak, and zone.

To get to camp #2, zone into Mount Zhayolm from the staging point , head to the Gates of Halvung at J-7. You'll need invisible to get past the clusters, and if it's night, sneak to pass the Ghosts. To open the gate you need to have the key item one gets from checking the Metal Plate ??? spawns. To get the quest, click the gates then find the active ??? that will give you the key item.

Now head to H-7 where the camp is. To reach it you need to pass sight aggro Trolls, and sight, and Job Ability aggro Flans, your targets. There is also one sound aggro Clot in the tunnel that your camp will be in. It's far enough back to not be an issue, but take care to not aggro it. There can be True Hearing Wamoura along the way. Give them some distance when passing them.

Once you are in camp, you can pull the Puddings to you with a ranged weapon. Since they aggro Job Abilities this is the safest way to snag them one at a time.
 

Amorphic Spikes can be learned in a Troll Mercenaries Besieged. You, a party, or alliance member must get the killing blow on the Fighting Flan for a chance to learn the spell. You do not need to have had engaged the monster yourself to learn the spell.

Amorphic Spikes can be learned in Salvage. You need the 1500 Assault points to enter, and to be in the right section. Your party should be able to handle them.