1. Buff up
2. Enter on avatar's weakest day
3. Run half way
4. Summon Radioactive Squirrel
5. Drink a Yag
6. Run up to battle arena outer ring
7. Assault
8. Astral Flow and Searing Light
9. Run back to halfway mark where you don't see Carby, maybe a lil farther
10. Hi-Ether
11. Wait for dead Carby
12. Resummon Carby
13. Searing Light
14. Repeat steps 9-13, running back and forth between the Entrance, Halfway mark, and Battle arena pattern.
15. Do not forget to drink more Yags.
If Astral Flow wears...
16. Wait for dead Carby
17. Resummon Carby
18. Follow the squirrel for the final showdown.
19. Attack the avatar with your best skilled weapon (hopefully staff) and Dia
20. Kill the avatar best you can
There were only 2 avatars that took more than 1 try to kill. Ramuh (paralyze's a bitch), and Titan (My first avatar fight). Takes 2-3mins, most can do it in 2-2:30mins. Or just wait till you're 90-99 and solo the pushovers. I was a 75 PLD/RDM (Rampart the 2hour) and a 75 DRG/RDM (Super Jump the 2hour) who visited the avatars regularly for money. Phalanx, SSkin, En-avatar's opposite element, and Bar-avatar's element. Not hard either way you do it, just don't forget to do it on the weakdays.
Eh, get it? Weak days? I made a funny.