I've done it!!
Ill share my experience as I think they could be of use to someone else doing it. I duoed this with a 56WHM but in all honesty, I count this as SOLO as all she did was /follow me and also die a few times on aggro. I died just the once right at the end with the HIgh Priest Yag, and Reraise wore as I hit the deck

I still got the crest though, and HP lost me 2kxp. Not too fussed.
1. Davoi.
Following the instructions on FFXIclopedia were easy, just made sure invis was up, thats about it. There are a few points in which you have to drop invis to check the Wall of Banishing or Ponds, but if you are patient you can wait until all backs are turned.
The only time it got a bit hairy was when the WHM got aggro, and ignored my shouts to Pull Back - then 2 others linked. I kept them busy while she went and got her orb done, then tele-holla'ed outta there with 50ish HP to spare.
The bit with True Sight NMs - easy as pie. They are right over the otherside of the room and you needn't even get close to them.
2. Beadueax
Even easier. Bring echo drops. Sneak all the way, Ruby Quadav's are easily avoidable again if you are patient. Instead of risking the gung-ho run for it strategy, just WAIT and they will eventually move out of range. The only tricky bit is when you have to run a fair way to silence yourself, and then sneak wears lol. I just let myself get cursed then and got the whm to cursna after, no biggie.
3. Castle O
This one is the trickiest. I went through the hassle of gathering passwords for statue - I'll tell you now, DONT BOTHER. Its easily guessable. At the end I told the WHM to wait by the door while I lit the torch, with the view of her letting me in from the inside. So I did just that, and ran in, gave the passwords and dropped, leaving me stranded. SO I had to hang around for a bit to find a random person to light it for me. Got to the statue - and the new game day had just started!! So Iguessed the password. In all honesty, its quicker to just run to the end and guess then to go to all the statues grabbing passwords.
The last bit with the NM - the way I was gonna do it was buff up - Proc/Shell/SS/Phalanx/Blink - Invis, run in, snatch crest and warp out while be beat on. But this time I had to raise the WHM first, and that plan didnt work out all too well. She had got her crest but died. So I had to buff up, raise, then go grab it, then do chainspell + escape. unfortunately I was about 12MP short of escape when I came to use it

Whilst I had hate the WHM managed to warp out.
So, all in all, success.
Thanks to everyone's suggestions!