He means the 20 solo cap fights...
Here's what I did, it's an almost 100% win, though a bit expensive.
First of all, if she is a taru this may not work. You can try anyways.
Supplies Needed:
decent level 20 or near 20 gear, I'd go with vit rings or hp rings also instead of mp rings. Even if you are galka, your avatar won't be out long.
A good battle pole, I used merc pole (level 18 windy CP item) and I sugguest it.
1 fish mith/cheifkabob/boiled crab/tav. taco. (any pld food will do)
10 high potions (you should have some left, but it never hurts to be safe)
1 yagudo drink
Strat:
When you are at the npc that warps you, starting looking for a high level pld,whm,rdm for shell and protect. I've waited an hour and a half for one on ifrit -_- Anyways, get a friend to do it if you can. Despite what people say, protect and shell 4 WILL last long enough. So if you can get them, get them. You need at least protect and shell 2. Don't worry about bar spells, but have it casted on you if you wish. While the mage is buffing you, quickly eat your food and cast aquaveil. (AQUAVEIL IS IMPORTANT)
After you done this, quickly enter the BCNM. Go up to the avatar, summon carby and have it attack. Now, almost every other strat tells you to have carby tank, don't do it. You should be up front with carby, meleeing. Don't let him tank, take hate from him. I usually cast dia,para,banish and it did the trick. heal yourself with mp best as you can since you will be losing it soon anyways Melee it this way for a bit (not too long mind you), I usually stop when it's hard to keep myself healed without(usually when if I'm forced to use a potion) or better yet ,when the avatar has lost a small chunk of his hp, maybe 1/8. I drink my yagudo drink and astral flow, and use searing light after this. The reason you want hate is because carby -CAN- be interupted from astral flow, doesn't happen often but it's happened to me. So you want hate on you while he's casting it. After searing light, the avatar should be at half hp or less, and should be on carby.
If you did it right, and if you have your summoning skill capped, the avatar should use his 2hour before he melees carby to death, use this time to heal yourself to full. I've never been killed by the enemy avatar astral flow, and this is the point the potions are for. You can win or lose, depend on if you can get carby out. So time it right for best chance, use potions to heal if needed, and pray you remembered aquaviel. Once you get carby out, use searing light again right away, and it should drop. That's right, a 2 astral flow drop. The only one who has not was shiva, but I managed to melee her to death anyways.
Hope that helps, I find this a lot more effect and less chancy then the stand away have avatar tank method, a lot more staightforward.
And excuse the typos.. so late here. >.<