Well, just kill it... ;-)

Seriously, he is not that hard if you levelled up enough. Check for Evrae in

http://db.gamefaqs.com/console/ps2/file/final_fantasy_x_eu.txt

Ciao,
Michele

Try to have everyone poison protected, and your fighters blind protected
(using armour), and use long range attacks. If Evrae inhales, then pull
back as soon as you can, otherwise you will be blasted by it's poison
attack (very nasty). However, if you spend the whole battle away from
it, then it will swoop back towards you, hurting you in the process. If
you have Underdog's Secrets, then using Rikku's overdrive will help.