After much experimentation, Thornmarch on Ninja is the fastest ARR trial to complete this on when playing solo, followed by Ultima Weapon, Garuda and Ramuh. Most of the other bosses have annoyingly slow or frequent phase transitions where you either have to wait or defeat adds.
Generally speaking, you should combine this strategy alongside passively filling up WT entries you would have already completed by doing your regular roulettes. This also lets you accumulate second chances that you need to retry this/whatever applicable trial.
I use this strategy to completely fill my WT when I just want the 9 seal reward to level my characters. Each completed WT provides half of your current level in exp. As it takes about 6-7 minutes to get about 5 seals, this is pretty efficient.
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