Better just to take 3-4 people. Alliances feed them the TP that causes you to wipe to those evil TP moves, smaller groups can more easily control circumstances and stop feeding TP if appropriate. If you do use a large group, bring BLMs and good RDMs to kill/control it fast. Large groups of melee are just asking for trouble.
If you look in the (lol)endgame forum alla recently implemented (should be in the menu bar on the left if you've not looked lately), there's a post talking about Yovra battle strategies that might help you out. I've trioed the Ul'yovras with RDM BLM BLM, and killed an Om'yovra with 9ish people on our way to limbus. Om's are 100% drop, and not really that much more annoying than Ul's. Just watch out if you do an all-mage type strategy, as they build resistance to gravity, sleep, etc over time.
____________________________Pogle | Gilgamesh Taru | Windy Rank 10 | ZM 17 | CoP/TAU Complete
75 BLM|WAR|RDM|RNG|BST|SAM|NIN|MNK|SMN|DRG | 56 BRD
Social LS: Icefire, http://becd.net/ffxi/forums
100+2 Alchemy | 59.0 WW | 40 GS | 25.2 Smithing
I'll try being nicer if you try being smarter.