Why not replace a BLM with RNG and just trueflight the thing?
*haven't done ambu yet*
Did several VD runs last night as both RNG and COR. The times I was on RNG was sadly without COR rolls, hence the final choice on my part to go COR. I think a RNG+COR setup could work wonderfully on this,
but-
When not stopping to do the 10k needles, the cactuar itself's evasion is off the charts. At first I tried strictly TPing on the Cactuar with the intent of using triple/quad shot to be able to leaden him every other attack (WS-tripleshot-WS, etc) and fill in any misses with quickdraw for dmg+TP. Well, with sublime+1, my midACC ranged set that while using Rostam(A)+Tauret that buff-less puts me around 1400racc on COR, entrusted Precision and two preludes...saw several full misses (4 shots, all missed) in a row trying to TP on the cactuar.
The solution, while slightly awkward, is to TP on the boss, saving WS's (magical) for the cactuar. We were able to slaughter the cactuar in one skillchain between SAM and COR of TP'ing on the boss, cactuar runs in for 1st 10k needles. SAM opens w/ Fudo, COR closed with a full 3k Leaden Salute, launched 2 Fire Shots in high TP set to reach 1k TP, and close double darkness with Wildfire.
The cactuar will be on the way out, and his evasion will have skyrocketed again, but that first Fudo will be during the "low evasion" window it has, and after that everything is magical and will land on him, giving you great WS numbers and a huge initial Darkness...the two Fire Shots don't really even need to be huge damage, just the TP for Wildfire (which even w/ DP over Arma was still 20k leading into a 30k+ double-darkness). First time we executed it, there was about 4% left on cactuar after the wildfire/doubledark, but a quick tripleshot on the boss and a free wildfire after that took care of it before a 2nd 10k needles.
After the cactuar was dead, swapped to Fomalhaut to abuse Light/Radiance on the boss. Easy 2 or 3 steps between COR and SAM. The SC damage was so good that it far out-performed Savage Blade spam (w/ naegling and Ataktos).
Once the boss is down, just pick off the adds one by one. A single 2 step chain either solo for the SAM or COR, or between them, was enough to kill any add in a single round.
Buffs for cactuar phase: Samurai/Wizard(Crooked) rolls, Wilt/Haste/entrusted Acumen, 3 song bard using March/Mad/Scherzo.
Buffs for Antican phse: Samurai/Chaos rolls, Frailty/Haste, March/Mad/Min.
A 4song bard would have made this just a cakewalk, able to maintain the scherzo full time. Its far from necessary, but WHM was being dualboxed by our tank and really made life easier.