I've found that, for the most part, Garuda is the way to go. I sort of got lazy and got Garuda and Titan and stopped until my set party decides to go out and get all the whispers. For the most part, parties expect the SMN to be a support class \ White Mage. With Garuda and Carby (Aerial Armor and Shining Ruby), you'll find that people REALLY WANT YOU!^^
Garuda is found in Rabao and Titan (if you want to bother), is in Bastok. Shiva is in Northern Sandy, Ramuh is in Mhaura (SP.), Leviathan is in Norg.
The strategy that seems to have worked for me is the one that is found on the Summoner.info page that was mentioned:
3xHi-potion
3xHi-ether
1xSteamed Crab (The HQ one.....if i'm wrong)
1x Yagudo Drink
And gear that cares only about defense. MP isn't an issue with the Avatar fights as you just run in, down the drink, eat the crab, summon carbuncle, (order here is up to you), send Carbuncle off to attack, Astral Flow (make sure you are by carbuncle when you do this), searing light, RUN AWAY!!!! Down a Hi-Ether, Summon Carby again, Searing Light again, RUN AWAY!!!!!! Down another Hi-ether, Summon Carby again, Searing Light and VICTORY!
It sounds easy but there are so many things that can go wrong it just isn't funny. Garuda's Aerial Rage one shotted me, Shiva just kicked my *** the second time I fought her, and Titan beat me down with Stone II. When i won I had 1hp with Garuda and 50 with Titan. I haven't even tried Ramuh (though I may do so today).
I hope that helps!^^