You know, it almost seems as if a small group is better for fighting these. From what little experience I've had fighting them (I've mostly avoided them) and the comments here, it seems that small focused groups of 2-3 have much better luck than larger groups killing them.
It almost seems like larger groups simply feed the Yorvas TP, which leads to them spamming their horrible TP moves.
So far, I've been able to easily take one down in a small group of 3 persons. This lead me to think that my larger alliance (which was farming after our Limbus run) should be able to school them without a problem. We warped to the Safehold, stepped out and found one near the zone to Sea Lion's Den (e.g., one that should have been on the low level side). With 11 people, including 3 BLUs, a SMN, a THF, a BLM, a WHM, a RDM and me as the NIN tank, we got completely and totally wasted by it.
We were doing very well, got it down to 50%, and were confident enough to jinx ourselves by laughing at how easy it was. Then we got hit with Charmga and the melees went on a rampage. The BLM managed to sleep all of us except for the THF (who evaded) and the THF promptly killed the RDM and BLM. When Charm wore, a couple of the UFO's floor pound move (I forget the name) managed to take out the BLUs. After this, erratic UFO moves (that seemed to me like hate resets) resulted in everyone else getting killed except for the SMN and I. We survived for quite a while longer (while everyone rested) but the Yorva had his crazy regen that was actually healing him faster than we could damage by ourselves. Needless to say, after a 30-40 minute battle, we lost.
Anyway, I need to test this more (as I've only fought these guys a few times) but it sure seems that smaller groups tend to have more luck. If anyone else has any data on this please post it.... stupid expansion has been out for a several years now yet there's scant real data online about how to actually kill them consistently (other than personal experiences, there's no real "this is how to easily kill a Ul'yorva" post that I can find.)