Esper levelling up question

[eppy19]

How do I get my esper's ranks up? Like i see on some sites that they have stronger attacks, do i have to use them alot or what?

[Drex]

From the GameFAQs FAQ on Espers:

A big part of Esper summoning is trying to trigger their final attacks (eg. Hellfire, Judgement Bolt, Tsunami). Final attacks are way more powerful than normal attacks, and affect a large area, but some final attacks are ridiculously difficult to trigger.

In battle, Espers do not gain experience. All experience goes to the summoner.

Whilst Espers do not level up themselves, their level follows that of the
summoner. So if my Vaan is lvl 40, any Esper he summons would be lvl 40.

You're seeing the final attacks of the espers in question, and to learn how to trigger each one you'll probably want to check out that FAQ. Otherwise, Esper leveling follows the summoner, like it said.

[Renmiri]

Exodus is the only one I had trouble with so far (among Belias, Mateus, Zalera, Adrammelech, Zeromus and Shemhazai) because the sadistic game designers made his special attack only be triggered when his summoner is under Immobilize and there's 30 secs left of his time. Hard to trigger!

I like to use Espers for when there are too many fiends around. They clear up the area in no time and you barely waste any potions or Phoenix Downs

[Dynast-Kid]

Exodus is the only one I had trouble with so far (among Belias, Mateus, Zalera, Adrammelech, Zeromus and Shemhazai) because the sadistic game designers made his special attack only be triggered when his summoner is under Immobilize and there's 30 secs left of his time. Hard to trigger!

I like to use Espers for when there are too many fiends around. They clear up the area in no time and you barely waste any potions or Phoenix Downs

Exodus's Deathblow (special attack), Meteor, can only be triggered when he is under Immobilize status for the last 10 seconds, not 30. Maybe that's why you have yet to trigger it. Your timing has to be PERFECT. I myself haven't triggered it.

And Exodus's Deathblow isn't even the hardest to trigger...