Storyline Acquired Espers
Belias The Gigas - Aries Zodiak - Likely the first esper you'll acquire, this one in the depths of the Tomb of Raithwall. The boss fight against Belias is one of the hardest battles you'll have fought up to that point. His Firaja Oil attack will leave you in big trouble if you don't have the Handkerchief's to get rid of it as he'll like you up with fire attacks. Have Shell and Protect up, and Berserk your best fighters (tanks) including Vossler. He is Velius from Final Fantasy Tactics, and you must use Belias in a story related event in The Feywood much later in the game. Belias is quite useful early in the game as he is the only esper you'll have for a good period of time.
Mateus The Corrupt - Pisces Zodiak - An ice based Esper, fought in the Stilshrine of Miriam. Mateus is near the end, right before the nethicite destroying sword. He (it's the fish, not the girl in front) likes Blizzaja, but you should like to use Shell. Of course Ice Shields are even better as it'll render most of his attacks useless. Towards the end he comes really fast and has no charge times on his attacks so your healing will have to be a step ahead. Still, an easy battle and an Esper that is probably better than Belias but may not be worthy of being used over two of your other characters right now.
Shemhazai The Whisperer - Sagittarius Zodiak - Shemhazai is weak against fire magic but tanking is your best bet. At this point in the game you probably have a solid strategy with Haste / Bravery / Berserk / Protect / Bubble and other positive status effects that you can cast to make this easy. Dispel her of course and her ability to change which elemental ability she has will make you not want to use magic anyway. She really is only dangerous near her end when her spells get off really fast and with Shock and Flare that could mean death for you. Disease and Silence can be a problem for you so be prepared with an item or spell to fix that. Fight through her Enraged state and you'll acquire an underrated esper. Her Soul Purge attack can do upwards of 50,000 damage on enemies if Devour Soul / Knot of Rust damage up to that point is high.
Hashmal Bringer of Order - Leo Zodiak - The earth based esper, who is weak against Wind. You have Reddas with you so that should help, and once you put Float on all your characters you can dodge all of his earth abilities. Dispel him before you do anything and prepare to use Dispel again at some point. Quakeja and Roxxor are his tough earth attacks. His puts up a paling to all attacks for a while so wait it out and keep yourself healed until you can finish him off. He is Hashmalim from Final Fantasy Tactics. Gaia's Wrath is his best ability for you to use but your summoner must have less than 10 % of their life for him to do it.
Famfrit The Darkening Cloud - Aquarius Zodiak - The water esper that you fight with Dr. Cid. This battle is hard because if you use the Berserk Tank strategy like I do, your Berserked characters will attack Cid who is immune to damage until you kill Famfrit. So, either don't use Berserk or just deal with it. Or you can Oil him up and use Firaga to take off 3 times the fire damage on him. Even with Shell up his Waterja spell will hurt bad unless you still have some Viking Coats laying around. Once he goes down you'll get him and you'll have to finish Cid off. As for Famfrit's use as an Esper for you, it benefits from the same thing Hashmal does; there aren't many water and earth attacks other than their's. At the same time he's the weakest level three esper around so his use will likely be situational for you (water-weak foes).
Adrammelech The Wroth - Capricorn Zodiak - Truth be told you can get Adrammelech really early in the game, if you're fortunate enough to defeat him. He was the esper Adramelk (Dycedarg) in Final Fantasy Tactics. Weak to Ice but thrives with Thunder. He is in the middle area of the Zertinan Caverns, you can't really miss it. As for the fight against him, he flies so your weapons must be long range. He likes to use magic like Thundara and Thundaja so Shell should be up and be prepared to use Regen to cure Sap if need be. The Shambling Corpses that appear around you can be an annoyance, but if you clear out an area and bait Adrammelech over there you should be okay. He'll use Perfect Defense towards his end so use that time to heal and kill the Corpses around you if there are any. As for it's use it's likely you'll acquire Adrammelech far after it can do any good for you. More or less the earlier you beat it the longer it'll be a functional option as a summon for you rather than an Esper that just matches up against Lightning-weak characters.
Zalera The Death Seraph - Gemini Zodiak - Zalera was Zalera in Final Fantasy Tactics, played by the Marquis Elmdor. Zalera can be found in the Barheim Passage but you'll have to do the sidequest to get the Barheim key first. Basically you enter Barheim and then go west to the screen previously inaccessible (near the Bloodwing hunt) and you'll reach a
Exodus The Judge Sal - Libra Zodiak - Refers to a FFTactics Advance boss and also to Exdeath of Final Fantasy 5. You must complete the Mosphoran Wasteland sidequest with the Craftsmoogles. More or less it requires riding a Chocobo around a secret path accessible through the east part of Mosphoran Wastelands and you are to turn the different shrines on. Eventually you'll open up a path to Exodus. The battle itself isn't that bad, he has no elemental favorite and you should Dispel him at all times. You cannot use Items but that's probably the easiest stipulation yet. Despite his no elemental favoritism he does like to use devastating spells like Scathe, and Flare. With a Bubble Belt or two you should be okay to survive these. He'll use Reflect but that can be Dispelled, and he'll put a paling up that you'll have to wait through. An easy battle for a poor summon, although it is useful for one thing. Exodus can cast Meteor, an attack that can take off 20,000 - 30,000 damage. Of course it requires there to be 10 seconds left of summon time and he must be Immobilized before he can cast it.
Cuchulainn The Impure King - Scorpio Zodiak - Queklain from Final Fantasy Tactics. Definitely my favorite esper in the game and a scary battle. You need to complete the White Mousse Hunt first to be able to get the key to gain access to Cuchulainn. That said you can do this rather early but Cuchulainn the Esper fight should be done around level 40 if not higher. With the Sluice Gate Key, light up the 3rd and 10th Sluice Gates. Now go to No 3 Cloaca Spur and light up the 1st South Sluice Gate. Return to the center and open the 3rd and 10th sluice gates, the ones you just used. Now, light up the 4th and 11th Sluice Gates and go to No 4 Cloaca Spur to close the 1st north sluice gate. Back to the center area one more time to open the 11th sluice gate and close the 3rd as you will have the middle two lit up. Go south now to the No. 1 Cloaca and you can now go through the gate at the south into the water. Before you do this, put your Haste / Bravery / Shell / Berserk buffs up as Cuchulainn looms. This battle is ridiculously hard as you will have a constant Sap on your party, and he can even cast Sap in conjunction to that to deplete your HP times two. Dispel him and Dispel him again if he casts more protective magic. You probably want Bubble Belts but Black Belts prevent against Disable which he loves to use. If you are immobilized then hopefully you have long range weapons or something to use to be productive while you wait that out. Foobar's will drop down from the sky (or the sewer I should say) and you'll want to wrap them up fast. If you have Berserked tanks they should take a second to do this. Watch for his Invert ability as if he gets this off one of your characters could die from Sap very quickly. If you have at least one character with a Black Belt you should be able to stay alive but without a way to protect from Disable this is one of the hardest fights in the entire game. As an esper he is a true brute for you with tons of great stats already on himself. He tends to lean to the physical side as his attack is highest when he has full power and you really will enjoy his ability to keep himself alive with Curaja and the enemy HP leeching ability known as Malaise.
Zeromus The Condemner - Cancer Zodiak - Zeromus was the final boss for Final Fantasy 4 (Final Fantasy II stateside SNES). You must acquire the Stone of Condemner from the acolyte at the top of Mt. Bur Omisace and then you are to take this to the Stilshrine of Miriam, where you should Save upon entry and then go to the Way Stone in the back of the first area. You will have the option of going to a new area so do so. Before you go through the door put some spells up as you cannot, I repeat, you CANNOT use magic in this battle. This made this very difficult for me and I expended more Elixir / Ether / Megalixir than I'd like to admit. The Dark Lord's can do some mad damage against you and will continue to spawn, which means lots of combos. Your best bet is to hope that your buffs like Berserk / Haste / Bravery last long enough to do a decent amount of damage on Zeromus. From there you should try to use Quickenings since you can't use magic anyway. Zeromus will attack with a good combo rate, and is downright lethal near his death with that. He also uses Graviga, a gravity spell that will have you running around to heal. Easily one of the toughest esper fights and one that will require lots of levelling up and maybe a little luck too. It's worth it though as Zeromus' Big Bang attack (from FF4) will do large amount of damage, with (Zeromus' Maximum hit points minus his current hit points) x 5. Needless to say later on with a Bubble Belt and with low current HP Zeromus can do a ridiculous amount of damage with Big Bang or Gravity Well. The danger of course is that Zeromus risks dying before he gets Big Bang off.