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The reason you couldn't beat Adremmelech with a long Quickening Chain is because he uses Perfect Defense when he gets to 1/5 HP which makes him immune too all damage for about a minute. Most/all of the summons after Zalera take two or three Mist Charges and are equally as useless as the others. A viable strategy is almost always better than a Quickening Chain or an Esper.
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Hint: Syphon to replenish MP rocks. Buy it at any airship
That is precisely what I liked about Espers / Summons: bring them out when you need something to kick butt for sure. I used them for that and my characters for all other battles.
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Belias, Mateus, Adrammelech and Zalera takes one Mist Charge.
Shemhazai, Hashmal, Zeromus, Exodus and Cuchulainn take two Mist Charges.
Chaos, Famfrit, Ultima and Zodiark take three Mist Charges.
Espers are useless. I rather have my Vaan fighting alone than have someone that require me to heal him/her everytime.
The only Esper I like to use is Exodus because he's a Libran. :p
But I'm a completist, getting them all is a must. :D