Once you get accustomed to how Tiamat is and people get over that initial "panic" phase of "OMGTIAMAT!!!", any good tanks should be able to produce enough hate to allow ridiculous amounts of SA'd WSs per landing. Within my LS we find 2-3 SMN is usually good enough damage with around 7-8 SA'ing melees to use WS per landing. We also DoT Tiamat until 20% ish. We use Requiem VI (w/flute), Bio II/III (air) w/ Dark Shot, Dia II/III (ground) w/Light Shot, Poison II, and if the Black Mages can, they keep two of the elemental DoT spells on. Don't ask me which because I am not a BLM, I just leave it up to them to choose them. In essence you can destroy Tiamat's regen capabiliy and do consistent heavy damage if your tanks are well prepared for this fight. Having the Samurai use TA+WS when SA isn't up but Meditate is adds good damage too with Overwhelm merits.
When Tiamat flies, swap the Samurais out for the same amount of Blue Mages for SA+Cannonball if possible. Be careful with claim errors though, although this should not be a problem if your tanks are consistently acting on Tiamat and holding high hate. We run two Black Mages full-time for stun (or at least that's what we used to use em for only), and they should be nuking when they can as long as they're not camping the Stun button (this applies to both ground and air).
So far the fastest we've been able to take her down is 36mins without any deaths or Spike Flails whatsoever. I think the best strategies evolve over time. Start out with mostly SMN and a few TA just to get a feel for the fight. Next time try adding the Blue Mages for flights. After that, increase the SA/TA and lower the SMN in main alliance. After that try full-party swapping SAM and BLU. Lastly, add the DoTs. Over time your tanks should be able to adjust to the hate management as it is slowly becoming more difficult with the above recommendations. Of course you can just completely manaburn Tiamat, but it's kinda boring and uneventful when done right. You can probably kill it in 20-30mins that way, but there's no challenge. Do what is best according to how your leaders feel and how your linkshell is setup.
Good luck on your dargons.
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