yeah, even w/o Atma THF treasure hunter works. Also did you finish any storylines? If you did you can probably get some
synthetic atma, some of them are pretty good, better than nothing.
I'd say your first goal should be to do some Mega Bosses and get some atma. Even just one or two, that way when your WAR is abyssea level you'll be ready to help out.
People are generally pretty decent about letting people join for atma. I know my LS has let a lot of people join up for the atma drop - so it could be worthwhile just going to where the mobs that drop the atma you want spawn and politely asking anyone who is doing them if you can have the atma. Also put it in your seacom when you are in the zone.