I usually never post here(since alla seems to be full of trolls) but once you've done a mini-battle you can't get another mini-tuning fork. The requirements for doing them are lvl 20 Summoner and you can only use Carbuncle to attack and you get no helpers(meaning that this is a solo gig).
You really only need to do these if you're missing pacts with the avatars and can't do the lvl 60 battles. It's pretty much useless otherwise because the only reward is "a pact with the avatar" but you can still do them if you've done a lvl 60 battle. My summoner LS does however like them for warping to cloisters for full on battles(since it's faster than running there).
actually, you can get another mini-tuning fork to do the battle again if you want to. the only thing it does not let you do is have a free warp to the proto crystal.
to the OP:
this is a solo fight, which is very highly recomended to do at level 20. the lower level you try to do this (minimum of 15) drastically lowers your chances of winning. the level difference between you and the avatar is very noticable.
the recomended strategy is the carbuncle kite/astral flow method. what this involves, is buying a few medicines. 2-3 yagudo drinks and 3 ether.
what you are going to do, is go inside and buff up as much as you can with what you have at that level. once buffed, go towards the upper part of the ramp. once there, summon carbuncle. once hes out its time to fight.
immediatly pop your first yagudo drink. it will make carbuncle cost less to keep out. now once carbuncle has hate on the avatar, its time to run as far away as possible, without the HP bar for carbuncle dissapearing. once carbuncle dies resummon him and again run as far away as possible.
repeat this until you have roughly 1/3 of your MP left. now its time to start astral flows.
what you are going to do, is when you resummon carbuncle after he dies, use your astral flow 2 hour ability. then when the avatar is in range of carbuncle use Searing Light blood pact rage. if done right you will inflict massive damage to the avatar as well as have 0 MP.
now is the time to start using ether. use 1 after each time you use searing light. now, the trick to using astral flow is to make ABSOLUTLEY SURE you have 2x worth of your level in MP otherwise you CANNOT use searing light. so make sure you have those yagudo drinks going.
one more thing to take note of, is if you are getting hit and trying to cast carbuncle, chances are you will lose. you will get interupted very often. the only thing you can do is pray you summon, otherwise you need to go re stock on meds and try again.
these battles are very unforgiving, but at the same time teaches you how to solo on smn which involes basically the same strategy. so i think it is a good lesson to do these fights.
i hope this info helps, and good luck with your fight.